The first thing to do is to make sure the valve from the LP tank is open. Even if it is, it may take a few minutes to get the air out of the lines. First thing in the spring, I always used to first try to light one of the burners on the stove as an indicator that the air was purged out.
If the stove lights and the water heater still won't fire after a couple of tries, the next thing to do is check the burner tube of the water heater for an obstruction. Spiders sometimes like to build nests in there.
Keep us posted on what you find.