Slang
Americans use all kinds of idioms, expressions, sayings and slang. Here are some sites to help you understand what people are saying.

This site offers a detailed list of expressions and idioms organized in different ways to help you find what you want: http://www.idioms.myjewelz.com/

Still can't understand? For specific individual slang words, try this site. The quiz format will help you understand and speak like a native: http://www.manythings.org/slang/slang2.html

Translation
If you need a translation try these free on-line translation services:

WorldLingo http://www.worldlingo.com/en/products_services/worldlingo_translator.html

BabbleFish http://babelfish.altavista.com/