The only reason to build a world is to give a story a place to live, when the story wouldn't necessarily fit into the world we already have. So to start I always look at what my story needs. Usually this isn't too hard to figure out. If the story takes place after the apocalypse, you need an apocalyptic world. If it's about a wizard that fights trolls, it will probably be in a medieval land. The story is key to starting the world-building process. My rule of thumb is only make what you need. If you have a cool detail about your world, don't shoe-horn it into your story. But if the story has a neat little factlet, make sure that it makes sense within the world.
The next thing I consider is my characters, or more specifically how my characters fit into and react to this world. What are these characters in this world instead of any other one? Plot makes a story cool, but characters make a story real. In my comic "Nuclear Family", I have five main characters. Each react to the world in a different way. Sage wants to fix, Millie wants to keep it from destroying itself, Grit wants to figure it out, Meredith wants to fit into it, and Oliver wants survive it. They all live in the same world, a world that's been blown to hell but nuclear war and god knows what else. I had a thought about fossilized cities. That lead to fossil bombs which lead to the idea for a character who's parents were fossilized.
The last thing I do is put everything together with the little details. How do people live? Who rules? What are the settings like? Things like that. By the time I get here I usually have everything pretty much nailed down. Sometimes building worlds aren't that involved, sometimes it get's extremely detailed. at the end of the day you just need to have fun with it and it will turn out cool.
