There should be pcie connectors coming out of the power supply, if there are not any you might have a modular power supply in which case it will say on the PSU where to plug the pcie power connector into. You then need to put the other end of the cable into the graphics card.
You have an HP, therefore you don't have a good PSU. That HP ships with a 300W PSU, it isn't going to have a PCI-e power connector, it doesn't have enough capacity to support one. You are going to need to swap out your PSU.