I personaly think slang is very funny and I like it but I agree with Neal and Power, it would be better if you first understand standard English before trying to use slang. Anyway the way I learned most of the slang I know was through songs and movies, they are a nice source

Strewth, that Power dude hit the nail on the head. The problem is that slang is very dynamic - it is always changing and can mean different things in different social groups. Sometimes, slang words change their meaning over time, and sometimes words of phrases can mean the exact opposite of what you think they might and unless you are talking with people who use those phrases, you can get confused very easily. You really have to pick them up as you go along. To learn slang is good because it is part of appreciating the language and idioms of other countries and communities. Most of it is not bad or offensive, but you need to be careful where you use them.

Why do you want to learn English slangs? It is better to learn proper English. Slang is a very informal language and is usually spoken by a particular group of people who know other well. Some people may get offended if you use slangs while talking to them. For example, we called a coward 'a chicken'.