Student or Learner
Is it necessary to study slangs ?how important are they?and,how much should I study them?more than usual words or the reverse?
Slang. You should study slang according to its importance to you. If you want to learn slang, study slang. But slang wont help you gain employment. Slang is also easier to learn because it is very common and often spoken with feeling and the emotional delivery makes it very easy to remember and duplicate. This site is a very good place to practise formal English. You can use slang else where, for example on your mobile.
These are personal opinions only.