There are many sites with idioms and proverbs discussed. It is worth reading them - starting with http://www.usingenglish.com/reference/idioms/.
For slang - particularly modern street slang - the urban dictionary is as good a place as any for finding out what people are saying: Urban Dictionary: Define Your World
If you cannot find the answer, try asking here - making sure you give the full background and context.
- For Teachers