I recommend starting with Solaris.

The Futurological Congress is also a lot of fun; it's one of his comedies, a satiric look at futurism.

His Master's Voice is advanced level Lem. It's less a novel than a fictional autobiography of a scientist, and while the subject matter is fascinating, it's a bit dry.

The Cyberiad is a classic, but very different and not really sci-fi. It's a collection of absurd, surreal fairytales about a pair of robots. It contains math puzzles and rhymes and lots of weird stuff.

Fiasco, Memoirs Found in a Bathtub, Tales of Pirx the Pilot and Star Diaries are supposed to be good.

I found Eden (one of his early works) to be a little juvenile and poorly written. It, too, is about first contact gone wrong.