I agree 100% with Keith. Once you understand the basic grammar and vocabulary of a language, READING BOOKS is the best way to learn new words. Reading news articles is excellent as well.
Some things to think about when you find a new word: Is this word still used/commonly used? Is this word specific to British/Australian/New Zealand/American/etc. English? Is it commonly used as part of an idiomatic expression?
Also, talk to lots of native speakers, and try to emulate the way they speak.