well, your power supply should have a whole mess of wires coming out of it. these are the power cords for your drives, CD players, fans, etc.
there will be many of these ends, with the white plastic pieces on them. there should be a yellow, red, black and white wire on each set, and there should be at least 3 sets of these coming out of the power supply.
now, if you have an old pc fan in front of that came out of a PC, odds are it still has the plastic connector on it, allowing it to plug into the power supply directly. if not, you have to do a little modification.
coming off of the PC fan will be two wires, a red and a black. all you need to do, is clip one of the plastic connectors coming off the power supply, so that you have access to all 4 of the wires behind it. you need to strip off the red and black wires, so you can connect them to your fan.
now, if you want to power the lights off the same power supply, i don't think that is possible with the existing wires coming out of it. see, it is a sort of transformer, stepping the power down from 120v AC (from the wall) to 12v DC (to power the computer)
there is one way that may work for you if you want to keep the number of wires coming out of the back to a minimum. in essence, you will be splicing the power cord for your lights into the power cord for the PC, creating one cord.
if you look at the PC power cord, on the end that plugs into the computer, it is a large, 3 hole plug. cut this wire about 4 inches back from the plug, to expose the inside of it and the 3 wires inside. there should be a white cord, a black cord, and a bare copper cord.
next, grab the extension cord that you want to power your lights with. this can either be one of those regular 6-foot cords, or a power strip surge protector. what you need to do, is measure out how much cord you need to keep inside the case so you know how much to cut. cut the male end (the wall end) of the extension cord off, leaving 2 feet or so on the other end, enough to run into your box. now take this, and splice it onto the PC cord you just cut. when you are done, you should have a pc cord, with an extension cord coming out of it up near the back of the PC.
that seem straight forward enough to follow? i know, i'm terrible at explaining things. let me know if that is what you were looking for. anything else, just.