There are two major ways to create a god NPC. The method depends on the purpose the character serves in the campaign. In either case, the god should help facilitate an adventure for the PCs. It is seldom a good idea to create a powerful NPC with a strong presence that fails to further the story.

The Golarion Pathifinder setting provides excellent examples of both methods with true gods and demigods, as described below.

1. Plot Device

I sometimes call this a "force of nature" NPC. A plot device god works best as some nebulous force that drives the plot by helping the PCs or complicating their lives. They rarely should directly interact with the PCs. The goal of any encounter with the plot device god should involve a quest, social encounter, or any other encounter that does not involve physical confrontation. A friendly plot device god might be a quest giver while an enemy plot device god might create a storm or a curse that can only be appeased with a quest.

Roleplaying Interactions

If the god must expose themselves to direct confrontation, then make sure all interactions are fun and interesting to the players. If a PC attacks then, don't just say "it doesn't work." Have something interesting happen, like the weapon going through them or blowing the fighter away. If you don't want this to happen at all, then have the god manifest in a way that makes it impossible for both sides to directly affect each other. Maybe the god appears as a reflection in the mirror or as a ghostly avatar.

Estatísticas

A god as a plot device should not have any actual statistics. Creating a stat block is unnecessary, patronizing to the players, and limits your ability as a GM what they can and cannot do. By creating a stat block, you create limitations of the character's power, which is not something you want to do when creating a character as an unassailable force of nature.

Exemplo

True gods in the Golarion are excellent examples of plot device gods. They have no statistics and rarely directly interact with PCs. True gods affect a plot by changing the circumstances or direction of an adventure. For example, in the module Clash of the Kingslayers, a god transforms a temple into a massive walking monster that the party has to go inside and explore in order to figure out a way to stop it. In the Ira dos Justos adventure path, a goddess offers mythic power in exchange for passing a test.

2. Active Character

A god as an active character essentially functions as a very powerful NPC. As such, you would stat and run them as you would any sufficiently powerful NPC. Such a god directly affects the PCs with their actions. However, it's absolutely vital that the PCs can interfere with or invoke those actions. Do not have an evil god enact an evil plan unless the PCs have some way to thwart it. Do not allow a good god fix a problem unless the PCs actively do something to receive that aid.

Roleplaying Interactions

You need to determine a good reason why an active god won't wipe out a party or make an adventure trivial. An enemy god might not be able to do so or find it impractical to directly confront the PCs. Maybe they don't see the PCs as big of a threat to directly deal with? A good god might be too occupied to directly do the PCs's work for them.

Usually the best way to handle it is having the PCs only interact with NPCs that work for the god. Maybe they have to face the evil god's minions. Maybe they can only speak to a good god by their servants. For a low level campaign, an active god should work more like a plot device god.

Only have an active god directly confront the PCs if you intend to change the direction of the plot (like a plot device god) or allow the PCs to directly affect the god.

Estatísticas

Follow the monster creation guidelines in the Monster Manual as you would stat any other creature. Don't just make up a list of absurd stats. Instead, determine what CR the god should be based on how high of a hypothetical party should be able do and then stat them accordingly.

Exemplos

Demigods in the Golarion setting are good active god examples. Unlike true gods, demigods in Golarion usually have statistics as a CR 20-30 creature. They're designed as powerful final opponents for a high level party or as rare allies called upon sparingly.

The Treerazer and Arazni are good examples of enemy active gods. The Treerazer is a demon trapped in a blighted forest that slowly tries to expand his influence. Arazni is a queen of an undead nation. Both of these demigods can directly affect the party without actually confronting them. A campaign using these gods will likely see the PCs thwarting their evil plans from a distance and taking down their minions until the campaign reaches a climax that confronts the demigods themselves.

Talmandor is a good example of an allied demigod as he's a force of good that likes to take an active role in a nation's politics and crusades. However, he rarely directly interferes with mortal affairs unless directly asked or if a massive national threat occurs.

Let's say I, a first-level fighter, want to lift a mountain with my bare hands. Do I roll a strength check?

No, because it's impossible.

If Zeus is walking the earth, and some lowly mortal decides to attack him, they have absolutely no chance of harming this immortal Olympian. As the DM, you don't have to give them a chance.

You strike Zeus' bare shoulder with your sword, but the blade simply glances off. He glares angrily at you and calls out, "What foolish mortal man would dare strike me?"

Gods are, nearly by definition, something more than the everyday mundane experience. In your world feel free to make them as strikeable as you want them to be.

There is a danger here (as mxyzplk pointed out). If you have a god walking the world, you need to make sure they don't crowd out the party's agency.

The players need to feel like they can do things that matter. If their actions are always overshadowed by a god's, the players will start to feel like they might as well not participate.

Your divinely-powerful NPC would work best staying out of the party's way.

So if the party is fighting orcs in the Red Desert, Zeus might be drinking with Thor in the north. If the party is negotiating with the western king, Zeus might be throwing lighting down on the Titans. Make sure Zeus isn't stepping on the party's toes.

The best way to do this is not to do it at all.

Take it from someone who's gamed for 30 years. Making your godlike, "Mary Sue" GMPC is always alienating to players. Even if they're "allied or neutral" they will hate them. Their reactions will range from silently thinking you're a goon to actively sabotaging your campaign to try to kill your beloved NPC.

The reason is that the game is supposed to be about your players' characters. Putting in some super powerful person, god, or whatever "for story reasons" puts your idea of your story, and that character, above your actual PCs. This makes the players feel like they're your puppets, as opposed to the stars of the game. If you want to play a character play in a game. If you want to write a story, write a story. If you want to GM a RPG, you should be concerned about your players having fun, not inflicting your pet NPC on them.

See also related questions about the problems with this - Meu GM criou um NPC Mary Sue. O que posso fazer para convencer a GM a torná-la menos poderosa? e Como usar um GMPC para o bem da campanha?

Even as a quest-giver who doesn’t go with the party, this is heavy handed. Why do you feel compelled to control what the PCs do? What if they do decide their “handler” is dangerous and needs to go? The fact that you’re envisioning a time that they decide to attack him and you need to totally prevent that means that you’re just wanting to control them.

You can do it with consequences - when the king hires adventurers, they could jump him and kill him but usually to be either butchered in seconds by his guards or be outlaws for life - but “No, you can’t” is the sign of a novice GM that is putting “their story” about the agency of the PCs. Don’t do it, they will not enjoy it.

Take a tip from fiction - the godly being who gives the heroes their quest never accompanies them on that quest (unless in disguise, never revealing their true identity or powers - and they'll need a good reason to be travelling with the party even though they're rarely of any use in resolving encounters; or sometimes in greatly-diminished form with part of the quest being to restore the god's powers whereupon they return to their divine realm). In fact, the heroes may not even realise that their quest was divinely inspired until much later in the story.

They appear in dreams and visions, or as "chance" meetings with mysterious strangers, talking animals, burning bushes, significant omens, etc to give praise or criticism for previous actions, to provide advice or cryptic clues, to moan about how time is running out, or just steer them in the right direction/give the next objective.

Very rarely they might intervene to rescue an overwhelmed party, heal or even resurrect the heroes, provide an unexpected opportunity to escape a bad situation, grant some kind of blessing (e.g. buffs that last hours or days rather than rounds) or artifact etc. They do not necessarily have to appear in person to do this.

Remember: if the god(s) could find/dispose of the magic McGuffin and/or save the world themselves, they wouldn't need a party of heroes and they wouldn't waste their time recruiting one.

You could have the god like character die and just respawn. In the TV show Preacher, the angels, when killed can choose to reappear near to where they died exactly the same as when they died. The god doesn't need to defend itself as it can create a new mortal form at will.

Plan B is intangibility. The players can see and hear the god but cannot touch or affect it in any way.

Plan C is invulnerability. Nothing the players can do will even ruffle the god's clothing. If the players attack the god, the god stands there waiting for them to stop.

Plan D is perfect regeneration. It doesn't matter what the players do, the god just pulls himself back together.

The key to being god like is the god doesn't bother to defend himself. Nothing the players do is a threat to him so he doesn't need to protect himself.

At the end of the day, you're the DM. It doesn't matter what the players plan and doesn't matter what they roll. If you say they can't hurt the god, they can't hurt the god.

Always know your character

God-like NPCs are possible, and @Cyrad's answer covers two basic types that you could have your god be.

However, I you like to talk about the "Story reasons" part. All story is made up of conflict. The vast majority of human conflict arises from two or more humans being at cross purposes. In a murder mystery, the game store owner thinks its perfectly appropriate to strangle and skin people that fit a certain profile: female, pretty, and an avid player of casual games. The police officer who just so happens to have a daughter that fits the description to a T disagrees, strongly.

Conflict involving gods, and thus stories involving gods, are only different on one head: more power. Take Troy Dennings "Crucible" - Cyric is on trial for killing a God in The Forgotten Realms.

Thus, you need to consider three things with your God: what are their goals, what is stopping them, and how are they being stopped?

Example: Antagonist

Say you want to bring in an evil God to pester and antagonize the PCs. Keep that word in mind: antagonize, which in most contexts mean to force someone to react in some way. An antagonist is someone who forces the protagonist to react. So, your evil god is forcing the players to react. One of my favorite ways to do that is to have the god, or its minions, boast about some unspeakably evil plan that the PCs would not want to happen (world domination, destruction of their favorite city, death to everyone they love, gentrification of the local goblin bohemia with Chipotle and Starbucks). Then, in the words of Maria Portokalos in My Big Fat Greek Wedding, you become the Neck, by making the player feel as though they made the choice.

Example: Ally

If you have a slightly weaker god that is on their side and is of the Active type, consider what it/he/she has to gain from the alliance. Is she just opposing her evil sister god, is she allied with other, slightly weaker god gods against this terrible, super-god like force, is she righting past wrongs, is she just trying to stir the pot and cause chaos in an otherwise stagnated multiverse, is she trying to tweak her goddess sister's beard by having them kidnap her mortal lover and hold him for ransom over a vat of molten lava?

Chink in the armor

E @mxyzplk is actually 100% right, you should not do this at all, at least not the way you stipulated. The truth of the matter is that players in these games are playing them because they pode actually effect the world, where in reality, their options are limited by circumstances and even more so by their own perceptions. If your perfect god is wounded by a sword, no one loses their job and no one's child become property of the state.

That being said, infallibilty is a complicated matter. Being invincible in combat does not mean the God can not be outwitted. So long as you give your PC's the slightest hint that there is an exploitable weakness in your perfect God (and there is ALWAYS an exploit), then their swords glancing off and their spells fizzling will be minor inconveniences compared to the drive they will have to defeat this god.

Oh, and say you have an Omni-powerful good guy God that everyone loathes: this is a good thing. Your god could either 1) change its behavior so that it is not hated, 2) take advantage of the situation and "antagonize" the PCs into acts of good, or 3) turn out to be a self-righteous, pompous, lazy good-for-nothing freeloader that is getting the mortals to do his dirty work (thus becoming the full on antagonist). And the evil god is just misguided (thus becoming the ally).

Remember: the story doesn't end because the PC's changed it. That's usually when it gets started.