You have a modular power supply. Do you have a box of wires from it? Look or which GPU did you have before?
You'll need wires that say "PCIe" at one end. Those will plug into your graphics card, it will likely be 2 seperate wires. One will be 8pin and the other you will make into 6 pin, with 2 extra expendable pins just hanging to the side (adjust as needed). There is also a designated place of power supply to plug the PCIe wire (if you need to add the wire that is, it's written on power supply which plug where, it will say PCIe and peripheral or something like that)