If you have the gas on, you should have a button inside the rig to turn on the gas water heater. That is what turns on the pilot light to the water heater and ignites the water heater. A light should light when it connects, saying pilot on - might not say it - and might not have that light - my rig does
If you're turning the switch and it's not lighting, try first lighting a burner on the stove (to get gas running through the lines) then try to start the water heater again. Try a couple times, if it doesn't work, then you'll need to check further.
If you've tried that and it's not lighting - then there are several videos on youtube about water heater maintenance - here's a link to one and on the right side of the page will have many others. I didn't know whether you have an Atwood or Suburban - so don't know.
http://www.youtube.com/watch?v=zWzs_v1PvtY
Beth
“Dare to live the life you have dreamed for yourself. Go forward and make your dreams come true.” ~ Ralph Waldo Emerson ~
"He who treasures the small things in life has found the path to true happiness"