[Namekuseijin]

Begin with a true classic: "The Time Machine", by H.G.Wells. Pretty short and sweet and yet retains the foremost rule of scifi: take a subject and speculate about it to the utmost consequences. He doesn't even incur into lame predictions for the next few decades, going instead well beyond current human civilization (thou by extrapolating social theories of his own time)...

It's also in the public domain and a free download.

After that, if you want something more hardcore, by modern authors and still freebies, you may try Accelerando, by Charles Stross or Afterlife, by Simon Funk, both very good reads, not lame space opera...