Make sure you have enough has in bottles, then turn gas knob to "off" position for about 10 minutes, come back and turn knob to "pilot" and push in and try to lite the pilot, it might take a few seconds, if there is air in the line, it will kind of make you lighter flame flicker but will not turn on, then you might have to hold knob in "pilot" position and push in for a minute or so, for air to come out, and at the same time keep trying to light pilot every few seconds.
What I do sometimes is point the lighter to the actual water heater flame thrower that heats the water, just next to the pilot light, and it will light on after a couple of tries and then you just wait a few minutes and the water will be hot, hope it helps, good luck!