A helpful tool: search for tweets containing a specific vocabulary word. You get to see a wide variety of uses, mainly informal, you get to see different connotations of the word (affectionate, derogatory, old-fashioned). Of course, it's difficult to get these connotations and contexts at first, but I'm sure it gets easier over time.

Also, a good informal/slang dictionary is useful (especially an online one, where native speakers can constantly add definitions, like Urban Dictionary).