There are screw terminals marked by the arrow. You need to attach wires with proper voltage to them. You could use AT, ATX or custom power supply.
Simplest, but destructive (for power supply) way: use old ATX (or AT) power supply you don't care about and strip the wires. Match colors of wires. Insert them into screw terminals and clamp down with the screws. Don't use orange wires (don't mistake them with red or yellow). If there are multiple wires of same color, you can join them together.
wirecolors.jpg
"Proper" way: buy ATX to AT adapter cable and remove AT connectors, insert wires into terminal block. Congratulations, you converted that computer to ATX. You can also buy miniature 12V pico ATX power supply if you want to use something smaller.
atconverter.jpg
Ideally you should check the voltages before turning everything on.