I think you might mean slang words. If so, this is very difficult to do until you are part of a culture because slang varies so much even within a country.
There are some basic slang words that are general to the U.S. such as "cool." But different social groups will develop their own slang words. To know what you want to learn, you have to know who you want to associate with. The more counter-cultural a group is, the more slang they will use, and more quickly their slang words will change.
This makes it very hard for English teachers or websites to keep up with current slang. Then, you end up learning slang that is outdated and you sound sillier than if you just didn't use slang at all.
Here is a website with almost 300 slang words, but you need to know that many of them are outdated. http://www.manythings.org/slang/
You might try watching MTV shows or other modern, edgy film to see what slang you can pick up from those.