When I switch my a/c on nothing happens there is a relay switch on the unit outside when you push it in the fan kicks on but when you release it shuts back off.

From your description you are missing the low voltage control circuit going to the outside unit. There should be a wiring diagram of the outside unit on the access door. You can use this diagram and the diagram on the furnace/air handler located on the inside of the blower door to find all the connections and wires for this low voltage problem. You will need a meter and know how to use it in order to track the problem down.

Some usual problem areas are.

A blown fuse on the interior unit circuit board.
A thermostat that is not set to cooling.
A broken wire going to the outside unit. (dogs and animals love to chew on the low voltage wires.)
A defective thermostat not sending the cooling signal to the outside unit.
A defective low voltage transformer not supplying the low voltage for the circuit.
A defective low or high pressure control in the outside unit itself.
A defective time delay circuit if one is used for your outside unit.
There can be other items that can also cause this problem so a good check of all low voltage circuits is in order.