You should not focus on formal English or slang until your English has reached an advanced level. If, for example, you go to study in an English-speaking country, you may need formal English for academic essays, and you may need to familiarise yourself with some slang expressions that you'll hear in your daily life there.
For the moment, however, you need to focus on STANDARD English. This is the straightforward everyday language that is used by English speakers 99% of the time. It's the English that you'll find in your textbooks, on TV, in TED talks, and in the English-speaking world in general. Not formal and not slang. Just ordinary English.
Of course, if you come across examples of formal language or slang as you're learning English, it does no harm to find out what these words and expressions mean and remember them. But you shouldn't focus on either of these extremes of language until you've mastered standard English.
You need to build a solid house before you start decorating it with fancy fixtures and fittings.