I ageee with Su.Ki., you need to learn the standard language first, and to high level, even before attempting to use slang.  Formel language is not useful in the real world since people don’t use it anyway, at least not on a day to day basis. In specific studies at a university, formal language might be necessary, but it’s not in ordinary speech. I always tell non-natives to inderstand slang, which is fine, but NEVER use it yourself until you reach a near native level of fluency, accent free or virtually accent free, and know the ins and outs of the slang first. Slang is very nuanced and very regional. And, if a non-native uses slang improperly , you’re likely to get either some funny looks, at the least,  or just be laughed at at the worst, and embarass yourself. Foreign speakers of English who use slang, even when used properly, just sound comical to a native listener. It’s an odd phenomenon that’s been used in comedy routines in the US for years. The foreign guy with the heavy accent who uses slang to try and sound native. It’s hilarious and doesn’t work out well for the non-native. Plus, you don’t need to use slang anyway. Just use the traditional words. I’m a native speaker of English and I never use slang, not to know knowledge. I have no reason to need it. So, understand slang, but don’t use it. 

There's no reason to focus on one or the other, but you don't have to wait until you're an advanced speaker to learn aspects of both. When you learn new words and phrases, it is a good idea to know if it is slang or formal so that you know what situation to use it in. My advice is learn whatever comes your way, but know how and when to use it.

Oh, and don’t confuse slang with idioms. Idioms are not slang.  Understand idioms and only use them when you know the language pretty well, and that’s fine. I use idioms all the time and don’t even think about it; but, I’m not that crazy about slang, so I don’t use it myself. Not that I’m aware of. 

Not to mention, using slang is not just about memorizing a word or phrase. Its knowing HOW to say it. Which syllable of the slang word gets a certain stress, what body language is used when the slang word is said. All of this comes very natural to native speakers but can come across completely wrong if not said properly and in the right context. Honestly, English is my first language and even I dont really use slang that often. Especially what the kids are using now, I guess that just means I'm getting old. Hahaha. Good luck. Dont stress over it. Learning a language should be as fun as possible .