You'll probably have Dune recommended to you. It's considered by many to be to Science Fiction what Lord of the Rings is to fantasy. My advice, given what you've written: Don't bother with Dune. Not that it's not excellent; it is. But it's more like fantasy with Sci-Fi trappings than it is pure Science Fiction, which is what you seem to be looking for.
Unfortunately I don't have anything really substantial to recommend, what little time I have for reading fiction being devoted to fantasy and historical fiction. But two works I've had recommended to me numerous times:
Amazon.com: Foundation (Foundation Novels): Isaac Asimov: Books and
Amazon.com: Starship Troopers: Robert A. Heinlein: Books
Hope that helps, enjoy.
