I took an extra atx power supply from pc, took the green wire, and the opposite black wire, (plug that plugs into your motherboard) and joined them, so the atx power supply has power without atx board.
connect the yellow wire from the power supply (12v) to the amps 12v+ and the ground to a good ground source. it can even be a peace of metal inside the cabinet, but be carefull where you mount it though.
Power the hole lot up if your comfortable with your connections.
Oh yes, one more very important thing, don't just buy any amp that you see, you must know the wattage of the computer power supply that you are going to use, example, if it's a 350 watt power supply go for any amp smaller than 300 watt because you might risk popping the amp, and if it's not going to pop the power supply it is surely going to over kill it draining it of power with every beet.
When powering on the hole lot make sure that the gane etc on the car amp is set to minimal. while the music is plaing set the gain of the amp up with a star screw driver until sound is max or to where you like it. You might hear a crackling sound, if so, set the amp gain down, becuase it is a sign of the amp pulling to much power from the power supply, but if you go for a smaller amp than what the power supply is, i don't think that will happen.
For the speakers.
I would recommend using 4 ohm mids, example if the amp is 300 watt, go for 100-150 watt mids, and if you are using a crt monitor you might want to look out for shielded speakers, because unshielded speakers might interfear with the crt monitor.
What you could also do is to connect two 8 ohm speakers in parallel, if i remember correct, then they should act as only 4 ohm, and if you want to use shielded 8 ohm speakers, go for old satelite surround system speakers, as they are shielded. (Magnetic field won't interfear with monitor).
Well, that was a mouth full, but i hope the information helps you or someone else.
Regards
Quintin