I usually start with the general feel I want the world to have, and I pick out how big I want the world to be, like a mental limit. I sketch out some basic characters, because I base my plots on the characters and my characters on the plots. As soon as I've got a basic character idea and a basic feel for the potential story (usually just a "These two characters will be like this and they will meet up like this for this type of reason"), I work on gods. I like gods in worldbuilding for the whole creationist thing and stuff. Plus, I only really do a lot of worldbuilding with fantasy. If it's science fiction- totally different concept. But with fantasy, I start with gods, and usually base them off of preexisting gods and twist things about. The current fantasy I'm working on has several tiers of gods, and it gets sort of complicated and I haven't named them all or anything. Too much.
Then I name the world, and either find/generate a map or make one myself. This time I sketched one out on Paint, edited it so it looked the way I liked it, added in basic topography and colors to symbolize the type of land, climate, etc. And after that I start working on the setting, with countries/empires/kingdoms and the people that live in them. I develop the actual ecosystems and environment as I develop the countries and their history. Technology as well. And as I do this I develop the characters to fit into the setting.
Usually as I do all of this I get ideas to clarify the plot and stuff.
And for science fiction, I usually start with character ideas and a generalized plot, and then I think of a time period. After I've got that, I start thinking about technology, how advanced it would be, and in what way, and from where I go with very general stuff. Usually when I do sci-fi I've got lots of space travel, so that's planets, basic history, basic terrain and stuff. No need to go in uber depth for every planet right? And then if there's another species, I usually work on that before hand. After I get a bunch of basic planets done I work on setting, human history, technology advancements, and then I work on detailing the planets of the main story the way I do with fantasy.