The relay itself is usually a small, square or rectangular component with several terminals. Your best bet is to consult your owner's manual. It will have a diagram showing the location of all the fuses and relays in the underhood fuse box. The diagram will likely identify the starter relay by name or by its function (starter).
If you don't have the owner's manual, you can often find a downloadable version online by searching for "2000 Ford Focus owner's manual PDF."