Slang

It is better to use it only in speech not writings. But if you are not a native speaker, then try to avoid it as much as possible because it is considered an obstacle that makes your learning process slower.

You can use slang in informal, casual speech, as well as informal, very casual writing.  Depending on your subject, it may be appropriate to use slang in creative writing.

You shouldn't use slang in professional/formal situations, i.e. during  job interviews, speaking with strangers and/or elders, among professional clients, etc.

it would be better to not use it, but if you want to use, you can use it just when you speak with your friends,because if you use slang with everyone sometimes will be impolite.