try watching english tv dramas, which are produced with what i would call natural english,such as eastenders,movies like lock stock. there is a wealth of free viewing out there on youtube also. try looking at videos posted by native speakers,as the dialects are varied as is the slang used in different parts of the uk, such as london v newcastle for example. also slang tends to change from gender and age group. so the best bet is to trawl you tube for english/american sitcoms and movie clips. failing that find an english language partner or more than one from different parts of the uk/usa. hope that helps.

There's no point in using "slang" if the person you are speaking to doesn't use it. There's no universal slang - every region has its own slang, and using slang in the wrong place (which is very easy to do) makes you look a little foolish. It's nice to know, but your chances of using it may be rare.

Watch MTV. All there is is slang.

All these answers are fine. I just wanted to add I wrote a notebook entry called "Common English Slang" not too long ago. I tried to focus on good examples that are used a lot.

Everything mentioned above + American dad, two and a half man, good show to learn crap.... slang words, etc