I just got a new bike, I went to this bike park and the thing i hit my back rim. Now it is all mested up, like it hits the back break pads and it hits the frame.

How do i fix it? Or do i have to get a new rim?

Take it to a bike shop and see if they can true it. If it is bent enough to hit the frame they are probably going to want to replace the rim or whole wheel.

Your bike may have too poor of a quality rim for jumping-
perhaps post on bmx for suggestions on h-quality rims.
The hub can be saved and a new rim laced to it.
This may be more expensive than replacing the whole wheel though if single speed as I presume.
The front rim is easy and cheaper to replace- also will get taco'd often if bmx- soo i'd suggest stock up front,
Spend some cash for h-duty rear, as your main weight impact is rear.

Dumb question : have you had the wheel move from not being tightened enough at the axle ? Can´t imagine that you could taco a wheel that much and still be roadworthy .