The power comes from the ignition on a red wire with blue stripe. Goes first to the NSS ($50) at the base of the steering column. Then to the "S" terminal on the relay ($15) on the inner fender near the battery.
If you are not getting power at the relay like Pinto says it is the neutral starting switch like Dave says. You can bypass the NSS and if the car starts, search through the threads here for ways to adjust it so it works correctly. If you are getting power at "S" terminal but the car won't start, be sure the relay is well grounded to the fender. Then put the key on "run" and bring some + power directly to the "S" terminal. If that starts it the problem was the relay.
If it still doesn't start check the tightness of the battery terminals or jump the battery using a running car.
