The official rules for aerial combat can be found here. They are, I warn you, fairly complicated, and can be a real headache to run. I personally hate them.
So much so that I’ve created an abstract flight houserule, which eliminates most of the “3D-ness.” It is admittedly abstract – there are corner cases where this flight rule cannot do things you’d logically expect a flying creature to be capable of – but it’s much easier to run, and makes flight less incredibly powerful.
For the record, flight at 1st level is rare; even flying races tend to start with “glide” or similar until they get enough HD. See the Dragonborn from Races of the Dragon or the Raptoran from Races of the Wild for examples.
The reason for this is that the importance of flight is almost impossible to quantify. In open spaces, flight can make for completely asymmetric situations: that is, cases where the flyer can attack with impunity, because those grounded cannot meaningfully respond. It’s such a big deal that flight is considered by optimizers to be one of the most crucial things for any character to have covered: if you don’t have a class or racial feature that does it, you need to find something else that does (the Feathered Wings graft from Fiend Folio is a great choice if one’s alignment is flexible).
My houserule limits this somewhat as well, by the way.