How do I light my water heater?

I would guess the sensor is rusted and no longer making connection. Your water heater senses the ionization of the flame through the electrodes where it sparks. (Senses in between sparks) the fact that you get flame means you have propane and the valve is working properly, so the fact it shuts off means it is not sensing the flame. I would unbolt the spark assembly and take a wire brush to the all the connections. Make sure the electrode is touching the flame, once it is lit. If that does not fix it, then the ignighter board sensing circuit is bad, at which point you can replace the board. If you want to confirm it is bad, hot wire the valve to on temporarily. But be careful to light it first and then hot wire the valve on, not vise versa, or you may get a large poof when all the extra gas lights :)