Well, do you know if it is a DSI (Direct Spark Ignition) and/or is it a combination electric/gas water heater? If it is a DSI, there will be a switch inside your trailer that should have a red light beside it. If so, be sure there is water in the tank and gas is on. Simply flip the switch and it should light for you.
If it has been a while since gas has been through the lines, then you might want to fire up a cooktop first to get air out of the lines and gas flowing before doing the water heater.
If you also have electric, depending on brand, there should either be a toggle switch on the water heater itself, or another switch inside near the gas switch. Again, be sure water is in the tank before turning it on.
If it is gas only with manual light, then you have to go outside to the water heater. There should be a knob on it for PILOT and ON. Turn it to pilot and light with a long lighter. Once the pilot is lit, then turn the knob to ON. It should fire up.