My central air unit will not come on. There is a wire that comes out from under my house that I assume should be connected to the unit. Where does it need to be connected? It is a wire that is split at the end. It is a red and white wire.

Check the voltage. I think, since the colors are red and white, if they are single strand bell wire in an insulated housing, they go to the contactor coil from the t-stat.

Could you please tell me where that wire needs to go inside the central air unit? I have no idea where it needs to go.

If it's a small gauge wire like telephone wire its your low voltage from the thermostat. You need to turn the power off to the fan coil and out side A/C so you don't blow a transformer. When you remove the cover on the out side unit you should see the rest of the wire and two wire nuts. Repair and splice, restore power and rock and roll.