I don’t quite understand in which context I use “别”, could you help me?
It’s used in negative imperatives, when telling someone not to do something.
别客气 literally means “don’t be formal” / “don’t act like a guest”, and is a common way to say “you’re welcome”.
别动! = Don’t move!
别生气 = Don’t be angry.
You’ll also see 不要 as a slightly softer, more suggestive way of forming a negative imperative.
不要走 = Don’t leave.
Extra!
别 is also used in some compound words with different meanings. It’s root meaning is to depart; to separate.
区别 / 差别 = difference
这两个有什么区别？= What’s the difference between these two?
别的 = other; else
还要别的吗？= Do you want anything else?
还有别的吗？= Is there anything else?
别人 = others; other people
别人都走了 = The others have all left.
告别 = to bid farewell; to say goodbye
我们告别了一个时代 = We have said goodbye to a era.
特别 = special; especially; particularly
我足球踢的不特别好 = I’m not particularly good at playing soccer.