how to biuld an ancient computer?

ok, before you ask im trying to biuld it for my keyboard (musical). I have pretty much every part you would need to biuld about 5 of them and I know how to biuld computers, I just have no idea how to make these old ones work. I have gotten one of the computers i made to display stuff on the screen but now when I try try it turns on and nothing happens. the cpu's I have are intel pentium, AMD k6-2, and an intel i486 DX. I have no idea what motherboards i have but all of them will fit these cpu's. I am using a (big) taiwon liton electric 200watt psu. so do I need a hard drive or is there an os or something biult in to the motherboard or what? let me know thanks.