Seconded: the Ender saga...and read ALL of them. Well worth it! Also, by Orson Scott Card, try Pas****ch: The Redemption of Christopher Columbus. AWESOME.
Also seconded: Asimov, Clarke, Bradbury and Heinlein (my personal all time favorite...read his books: Starship Troopers, The Moon is a Harsh Mistress, *Podkayne of Mars, or Stranger in a Strange Land...and if you like his style enough, find a copy of Time Enough for Love).
Philip K. Dick (Do Androids Dream of Electric Sheep?)
Tom Holt (humorous scifi, along the same lines as Hitchhiker's Guide to the Galaxy...get either Blonde Bombshell or Life, Liberty, and the Pursuit of Sausages)
Poul Anderson (try Tau Zero)
Ayn Rand (Anthem, specifically...yet even Atlas Shrugged has a bit of a sci-fi aspect to it)
Edgar Rice Burroughs' Martian Tales Trilogy
Michael Crichton...anything, really...Timeline is good sci-fi
And that's all I've got for now. Let me know if you try any of this stuff out and like it.*