I just bought a 12vdc 40a relay from radio shack got like $5. here is how to wire a relay up:
from the switch it goes to the coil on the relay, the other end of the coil in the relay needs to be wired to ground.
then you need a heavy gauge wire (10awg should work) from the battery to one side of the switching end of the relay. the other end of the switch in the relay goes to on of the spade terminals on the starter.
here is what happens, you turn the key to the start position. That energizes the coil and it switches the relay. now voltage from the battery can go to your solenoid on the starter.
Its fairly standard on the numbering for the leads to the relay but I don't have them infront of me right now. I hope that helps, i tried to "dumb" it down a bit for you non-geek types
PS: doug posted a beautiful picture that shows exactly what i said, awesome doug