Corny_Stewart :
are you sure?? I can't just get the cables and plug it into my power supply
The power supplies that come with pre-built computers are usually garbage(And likely not even good enough to power a separate GPU) and I doubt yours came with a modular one... I don't get why there wouldn't be cables...
If it is a modular one (Basically means the cables are removable btw) the cables should come with the power supply, or in this case with the whole PC.
If for whatever reason it really is a modular PSU with no cables the you need to buy a new PSU (Power Supply Unit).
It's possible to buy cables separately, but that's generally something only modders do and it's expensive. (Unless you use regular cables and buy them from like ebay or whatever)