Off basically means "not on", but it can also mean away, apart, from, and so the list goes on. If you use it to make phrasal verbs, then I'm sorry there's no simple rule. You need to learn phrasal verbs as you encounter them, and then later you might get an idea of how they work. By the way, phrasal verbs are not made from just any verb. There is a pattern, but you should learn them first and then work out the pattern. If I try to give an explanation, I'll just confuse you.