Open it up and look inside.
You're looking for...
That's a PCI slot, generally speaking, you don't want to use that for a video card unless that's all you have.
Or this...
That's an AGP slot. They're quite cheap to find cards for, and most likely, that's what you'll have in your computer.
Or last...

A PCI-E slot. Notice the difference, where the slot's break point is. On a PCI-E slot, it'll be towards the back of the case, on a regular PCI slot, it's closer towards the front of the case. PCI-E slots also generally have a locking mechanism, where regular PCI's don't.
Post back whichever you have.