Until you are quite proficient, it is a best to speak the English taught in classes.
It is good to learn how to _understand_ colloquial, conversational English. You can do this by listening to English radio shows over the Internet, podcasts, YouTube videos.
When speaking, the important thing is to communicate clearly. If you have a slight accent or slightly incorrect grammar, your listeners have to work to understand you. You can make things easier for them by avoiding anything that would surprise them.
Sticking to standard, _predictable_ English. You can be friendly and informal with your tone of voice and body language.
Anything informal is culturally complicated. It takes long experience to know what kind of language to use in each social situation. I am learning Spanish. I was taught to answer "Gracias" ("Thanks") with "por nada" ("It was nothing.") One day, talking to a language companion, I answered "Gracias" with "no hay de qué" ("There is nothing for which [thanks are necessary].") She burst out laughing. I said "Did I say something wrong?" "No," she said. "Was that rude or impolite?" "No." "Why did you laugh?" "It wasn't the kind of thing I'd expect YOU to say."