You don’t say if you’ve been trying but it not been working.
So first thing is to find the cut off valve it may be in off position so no gas getting to the burners.
Next is where the control panel for the water heater is either its on the seat frontage where the heater is or on the main control panel (Zig make mostly) each have a switch and three light lenses.
Green, Red, Amber.
Green lit , the burner lit successfully and will stay on until you turn off the heater.
Red lighting failed (fail safe). Wait 10 secs try again.
Amber shows leisure battery current low so won’t operate the heater .
If you have been trying and the gas is on, the leisure battery fully charged, and it still doesn’t light then I’m afraid its time to call an L.P.G (Low pressure gas) registered gas safe engineer to check it out , the worst scenario is if the gas module needs changing.
Phone around for best quote and they will test all appliances and give a safety certificate, and will estimate any repairs required.
You have to remember the heater is on auto ignition with gas I believe temperature controlled
Bob UK
May 2014