1) Start first with any good English "desk dictionary." I really like the American Heritage dictionary. English dictionaries will not have true slang, but they will contain many common colloquialisms and idiomatic phrases, labelled as such. It is also available online: https://ahdictionary.com For example, if I look up "crunk," I see: crunk (krŭngk) adj. Slang Wildly excited or intoxicated. n. A style of hip-hop music originating in the southern United States, characterized by electronic sounds, simple refrains, and a high-energy, heavily rhythmic beat. 2) Wikipedia, although it is an encyclopedia rather than a dictionary, is very good on this kind of thing, and is usually thorough and accurate. Thus, http://en.wikipedia.org/wiki/Crunk is a long article about the musical style. I do not like or trust the "Urban Dictionary." It can be helpful but I feel it is unreliable.

http://www.urbandictionary.com/ can be a useful reference, but be careful. Anyone can edit it so many of the entries are jokes. Many entries can also be offensive, out-dated or limited to specific region.

All most for all English dictionaries show you vocabulary with idioms, informal, formal and slang basically.