“Are you trying to start something?”
A stranger in a bar is making mean jokes about you. You think he wants to fight you, and you're getting angry at him too. You threaten him this way.
Are you trying to start something?
"Start something" is a slang phrase for starting an argument or a fight:
He came up to me and tried to start something. I was like, "Whatever!". I just ignored him.
If someone is acting in a way that seems like it might cause an argument, then you can say that that person is "trying to start something". For example, family members might have a conversation like this:
A: What are you doing? You're doing that all wrong.
B: Are you trying to start something?
A: No, I'm just saying, that's not the right way to do it.
B: I don't want to hear it!