You need a power supply that is at least 450w to power the card and your computer, one x16 PCI Express slot to slot the graphics card into and you will need two 2x6pin PCIE connectors on the power supply to plug into the graphics card.
Also if you were confused about what Michael Hurtle said about the card needing two PCIE slots, he meant that the card is 'dual slot' sized. In which means that when the card is plugged into the PCIE slot the heat sink (the plastic/metal cover on the top with the fan) will cover the second PCIE slot due to its large width.
I also recommend that you have at least a total of 8GB of RAM in your computer and a CPU to match the cards power if you want to game comfortably.
Regards