Ahhhh, the age old issue; why are there so many goddam space marines!
Most play pretty much the same, but not all. There are differences, but I think (as I did) the easiest solution for picking an army is choose the one you like the look of, read up on how they play and if you like the sound, get a 'start collecting' box. If you then don't like them after having a game or two, there'll be someone who will take them off your hands.
If you need inspiration, check out places like lexicanum, 40k wikia, and (NSFW) 1d4chan. They all provide decent places to read up on fluff, with a few piture resources attached.