how to use a computer power supply for 12v and 5v

Hi all...Im trying to figure out a way to use my old computer power supply to
give me the 12v and grnd and 5v and grnd i need to power the stepper
driver board i have...My question is how do i get it to give me the power? It
is not hooked up to anything...just got the connections dangling...When i
plug the power cord in to the wall, using my test meter i can find only 1
wire that will give me the 5volts i need, but none have the 12 volts...What
do i have to do to get 12 volts also?.......I tried searching this topic and did
not find much,so hopefully this not TOOOO basic of question that has
already been answered......Im new to this machine assembly stuff......but
this board has been the BEST place....Thanx for any and all help ! mike