Well, it depends on what you're interested in. For Space Opera -- big-scale stuff, interstellar shenanigans, galactic empires and the like, there are classics such as Isaac Asimov's
Foundation series, and Frank Herbert's
Dune books (although I don't think that any of the later books in the
Dune series were as good as the original; still, the first one is well worth reading). There Iain M. Banks's "Culture" novels, too: start with
Consider Phlebas and maybe move on to
The Player of Games (a favourite of mine) and
Use of Weapons before moving on to the weirder
Excession. The stand-alone
Against a Dark Background is good fun too, as is
Feersum Endjinn.
For less epic, more gritty and often nasty cyberpunk stuff, Neal Stephenson's
Snow Crash is a laugh (the main character is called Hiro Protagonist, and he delivers pizza for the Mafia).
The Diamond Age is good too. William Gibson's
Neuromancer,
Count Zero and
The Mona Lisa Overdrive are great books. Bruce Sterling's
Schismatrix is an exellent future history. Michael Swanwick's
Vacuum Flowers, and the novella
Griffin's Egg, are brilliant, although you might have to look for them second-hand. Charles Stross produces some fairly mind-bending stuff:
Accelerando is one example. Ken MacLeod's novels are good, too: I like his Fall revolution series,
The Star Fraction,
The Stone Canal,
The Cassini Division and
The Sky Road. Quite political, though.
Also quite political, and quite "hard" SF, are Kim Stanley Robinson's Mars books, about the settlement and terraforming of the planet in the near future. And of course there's George Orwell's
1984, and Aldous Huxley's
Brave New World... and speaking of classics, there's H.G. Wells's
The War of the Worlds and
The Time Machine, too.
Jack Vance writes excellent SF, as well as fantasy:
The Dragon Masters (a novella) is great, and his latest (possibly last) books, set in "the Gaean Reach" are good fun even if there often isn't much of what you could call a plot...
Ports of Call and
Lurulu are really just about trundling around the stars in a tramp freighter. Of his fantasy stuff,
The Dying Earth books are very, very good, and very funny too in their way.
M. John Harrison's
Light and
Nova Swing, and
The Centauri Device, are bogglingly good, although (no offence!) if you're 13 I'd leave it a few years! I'm pushing 40 (with my face) and I had to take them slowly...
I still don't understand
Nova Swing, although I definitely enjoyed it.
David Brin's
Uplift books, just about anything by Arthur C. Clarke, Ray Bradbury's short stories (
The Illustrated Man is a belter of a collection), Larry Niven's
Tales of Known Space books (
Protector,
Ringworld and so on), Kurt Vonnegut's
Slaughterhouse 5... a whole bunch of other stuff. Oh, Alastair Reynolds books: quite "hard" SF again (no faster-than-light, for a start), but good when you get into them.