Hi, and welcome to Tom's.
First of all:
How to Ask For Advice
And secondly: You pretty much need a new PC. I'm not even sure if it has an expansion slot for a GPU, and if it does, it'll have to be low profile which means low end card, and on top of that, you probably don't have much power to spare with the PSU it comes with. If you really don't want to consider a new PC then you're going to have to open that computer up and see what you've got.