I'm going to steal my money back
Why are you stealing money back?
This girl is causing me to do it.
It's reasonable to want money back that you are owed. People should always pay back their debts. However, stealing can get you in legal trouble.
I'm going to get her.
Confronting her in an attempt to have her pay you back does not sound like a good idea. It's better to talk calmly or take a legal route.