The way I see it, we can best do this one of two ways: start at the bottom and work up, or start at the top and work down. 

The first option means begin with the world itself; what kind of environments there are, what kind of plants and animals, then move on to what kind of races live there and what sort of cultures they have. It's also possible to do it the other way around, and build a world to fit a race of people.

The first thing I'm going to throw out there is related to the world itself, and that's the possibility of a planet orbiting a binary star. =) That's something I've been wanting to do for a while.