Hello Joy -- and welcome to MobileRead. Since you say you like Science Fiction, you definitely should check out
Baen's free library. Baen has released over 110 Science Fiction and Fantasy titles which can be downloaded in many formats.
The library (upload forums) here at MobileRead contain lots of classics, including some of the earliest science fiction such as
The Log of the Flying Fish,
20,000 Leagues Under the Sea, and
Lord of the World. You'll definitely find some of the masters: H G Wells, Jules Verne, E E "Doc" Smith, and H Beam Piper. Some contemporary authors have released their books under a Creative Commons copyright or have explicitly placed their books in the library here. Check out the books by Steve Jordan, Jeffrey Carver, and Cory Doctorow, among others.
Welcome. Welcome, indeed.