I just caught my dad cheating and I don't know how to tell mom about it so I've been keeping it to myself for a week now. What do you think I should do?
That's a hard one. Let me think. What happened when you caught your father?
I caught my dad and his girlfriend making out together and I started crying
This is understandably a difficult situation to understand, however, it is not your responsibility to let your mom know. There's a possibility that both of your parents are aware of what is happening in their relationship. Talk to your dad about  how you are feeling and that you want your mom to know what you found out.
Why? What's wrong with me.
Don't be afraid because what you are experiencing is normal, so there is nothing wrong with you. Your well-being needs to be addressed, and the first step is to call out your dad's infidelity because he is in the wrong, not you. There is no reason to have this burden on you, so you should definitely make it known that you are aware of what is going on.