Science fiction worldbuilding depends heavily on how "hard" you want the sci-fi ... as in how plausible you want the science. If you want it really plausible, the best way to do it is to look at actual science and take some intuitive (and fictional) leaps.

If you have a library nearby or can spare the cash to buy it, Orson Scott Card wrote an invaluable book about how to handle all of the things you're talking about. If not, here are a couple links to get you started (these are all places I've gotten good stuff for the sci-fi I'm researching). A google search on "science fiction worldbuilding" should yield some good results too.

Fantasy Worldbuilding Questions (fantasy and sci-fi intermix alot, so still useful)
Interstellar Travel on Wikipedia (and pretty much everything under See Also)
Universe Today is a great site for the actual science of space as it stands now
Space Cynic will give you an idea of what the problems with different space ideas are, so that you can deal with those problems in your world.