You have to speak with native English speakers to learn that. Also be aware there is not really a general 'English' slang you can learn. It's very regional, so you may learn from someone in the south but someone in the north wouldn't really understand the slang. They would probably know of it but have their own slang to mean the same thing. i know this as a northerner living in London and occasionally saying things that souther friends don't understand or find funny!

Youtube will give you a good idea. Slang changes with what is trendy.

Thanks Paul , su.ki ,and everyone my idea about slang was totally wrong Thanks for all

Paul is right, Aya. You have totally the wrong idea about what slang is. Slang is NOT everyday casual speech. Many Arabic speakers believe that English slang is the equivalent of the regional dialects which you have in your language, and that the English you are taught at school is somehow not the 'real' English. This is not true at all. The standard English which you learn at school is the same language which we all speak in our daily lives. As Paul says, slang words and expressions belong to subgroups - London market traders and Texan truck drivers might use their own slang when they're talking among themselves. It would be ridiculous for Paul or for me to try to speak in the slang of these people, and it would be even more ridiculous for you to do so.

I suspected you didn't know what it was. No, that is not correct. Slang is language that is very informal, and is typically restricted to a particular context or group of people. Different subgroups have their own slang, and by using slang you show you belong to a certain group. Focus on learning standard English, that is what you are hearing in songs and movies. Slang varies greatly, not only between countries but within countries as well. Knowing slang is useless if you do not know standard English well.