My mom opened up a credit card in my name without telling me and now I'm really mad about it.
Why did she do that? How did you react when you found out?
I think that my mom is just a thief.
What you're mom is doing is a form of fraud and it is extremely bad to do, so it is right for you to be upset. What you need to do is tell her that she should close it immediately or else you will alert the authorities.
I can't close it myself since she has the card.
I am sorry that your mother did that to you. It is upsetting that somebody who is supposed to care about your wellbeing did something to hurt you. You do need to look out for yourself and report it.