I recently install a rheem furance with a relay and transformer on a metal plate with multiple wires coming out the back I need helP with this

Was this a very old furnace?, if so I`m thinking you are asking about a fan center, are these wires you mention still hooked up? On the transformer it should be labeled,, r,, w,, y, g, sometimes you got to look real close to see the markings, you hook to each post just like you would if you was wiring thermostat to a circuit/control board

No when I hooked it up only thing that was running was the fan motor I replaced the gas valve but I am not getting 24 volts to the gas valve either

Yes the furance is. Very old