Ah yes, the Gods of Gods and Heroes. Where would we be without them? Just read below and you might find out ;)

How many Gods do I get to choose from?

There are 12 gods in all. You, however, will get to choose between two of them at character creation. Which ones are available will depend on your class.

How long do I have to wait to start serving my chosen deity?

Not long at all. Very early in the game you find out that...well, don't want to spoil the surprise, but you do start serving your chosen deity very early in the game.

So what do I get for my loyalty to them?

You mean other than some really cool, godly powers?

Yes, other than that.

Minions! Each God has a unique minion they will eventually offer you (Jupiter grants a Minotaur =)

How do I use the Godly powers I have access to?

Same as any other ability, but it does cost favor to do so.

How do I gain favor?

The same ways that people did it back in the day! Make offerings to your deity, and, in the case of Priests, mark a target for your God. If the target dies while marked, you get favor! Neat stuff =)

Does choosing one deity over another change the game?

Well, outside the different powers you would have, there is the matter of the story arc. Each of the gods is unique, and will send you on a different path, so there will be some variety in the stories.