Have a look inside it and see what it has. It might have a brown coloured slot which is an AGP slot. If you do a google search for the image of an AGP slot it'll help you identify it.
If it doesn't have a AGP slot then all you can do is add a PCI slot graphics card. This is not the same as a PCI Express slot, the two are completely different. Your board will have more than one PCI slot, they are the white ones at the bottom of the board. Theres very few PCI graphics cards out there and they aren't that great, the ones you can get anyway.
Failing that, have a look at your manual and see if it mentions it.