All modern discrete GPU's use the PCI-Express x16 slot found in most, if not all modern desktop computers. It looks like this if you are having trouble. Also, what is the make and brand along with total watts your PSU is? Also what is your budget so that I can help you select the best GPU for your money.
This is a PCI-E slot a GPU uses. Yours can be any color, as long as it's the size of the black one in the picture, the GPU card will fit.