The first rule of slang is:

DON’T USE SLANG. 

OK that’s not exactly true... 

It’s ok to use well-known slang. Because A) Everyone knows it and B) It doesn’t say anything about the kind of person you are - because a lot of people use it. 

1. Slang is TECHNICALLY vocabulary that is used by a small group of people and it’s not understood by everyone. So if it’s really slang, most people are not going to understand you. And for the people that DO understand - if you use it out of context or incorrectly, you look like a moron. 
2. Also, the world is very complex, very diverse, especially in the US. The US is more socially complex because of how diverse it is. There are a lot of smaller social groups that make up the larger culture. But if you don’t live in the culture, you don’t actually know what it means to be a part of any particular group, or to be associated with that group. Like stuff that looks cool in the movies might not be cool in real life. This is like if I learn Japanese from watching Anime. I’m going to talk anime-Japanese to normal people and they are going to think I’m an idiot and also rude. In other words, it could make you look bad to the people that you are trying to create relationships with. 

How slang works…

What happens is that over time, some slang terms become part of the mainstream vocabulary. So maybe the dictionary still calls it slang, but technically it’s not slang anymore, because everyone knows it and uses it. For example: The dictionary says that the word “cool” is slang, but it’s not technically slang anymore because everyone knows it and everyone uses it. It’s part of our everyday vocabulary now.

Here is my point. I teach practical, everyday English. If I teach you slang words, it will be because they are so widespread and common, that you actually need to know them, because everyone is using them. I’m not teaching you how to talk like a 15-year old.