If you want to communicate with people from all over the world, they may not understand the slang terms. If in future you change your mind, you'll be glad you learnt formal English, as it's hard to change bad habits.

Obviously, the best way to learn a local dialect or slang is to spend time in that locale. If you want to learn American slang, a few months in Los Angeles, for example, would do the job. If this is not an option, I think you could gain a lot by watching television shows. Not the news so much as 1-hour dramas (like ER) or 30-minute situation comedies (like Seinfeld). If you can understand these shows, I would say that your listening skill for American English is pretty good. There are a whole different set of television programs for learning English spoken in the UK. If your goal is to learn a more international form of English, I would think international news programs may be the best (BBC, CNN, etc). Just a few thoughts.

I kinda agree with maky1117. It also depends on how much do you plan to take to learn the language. If you can wait, then just focus on learning FORMAL english, and the slang will come on its own, when you hear and read people use it :)

Your first priority should still come for formal English. It is like the relationship between adult and children. Adult is mature than children, so formal English is more systematic than slang. It is all right you learn and use some slangs when in a casual talk with your friends, but least include them into your writings. All in all, learn them separately and try to avoid messing with each other.