Things that may help.

http://www.acronymfinder.com/

This tells you what acronyms might mean. Before I had this, I had no idea what AFAIK (as far as I know) or LMAO (laugh my a** off) or anything meant. This is easy for me to use and helps pretty well.

http://www.idiomsite.com/

Warning:Web page colors might be triggering. It’s bright green over black with blue at the sides.

This lists many American idioms. Idioms are in yellow, explanations in white. The letters at the top are clickable; you can click them and they’ll take you to the letter you’ve chosen.

Urban Dictionary, Wikipedia, and TV Tropes can also be really helpful.