Turn your gas off and remove the front access panel. Check for spiderwebs around the oriface fittings and the spark igniter.
This will all look like the gas assembly of your bbq grill. There are several tubes that the gas runs through. At the beginning of the tubes, there is a header pipe, in which the oriface fittings are located.
Then look for the spark assembly that looks like the spark igniter of your bbq grill. With the front panel still removed, don't turn the gas back on yet, but see if the spark igniter is sending a strong spark. If not, check and make sure the igniter wire is not too far away from the metal that allows the spark to ignite the burners.
Once you confirm this, turn the gas on (with the front panel still removed) and see it the unit sparks and starts up.
MAKE SURE YOU HAVE PROPER VENTILATION AND BE AWARE THAT WORKING WITH GAS INVOLVE HIGHLY COMBUSTIBLE CONDITIONS!
If you are able to confirm these, and you're not getting the heater to start up, you may have a problem with one of the safety components (high limit switch, water pressure switch, fusible link, etc). Most of these you can short to see which is not working properly, but unless you are comfortable with working with eletronics and a highly combustible gas, you may want to have a professional pool company or the gas company diagnose it for you.
Gas heaters can be, and are very dangerous if there is a problem and you don't know what you're doing, so please be aware there are some inherent risks involved if you decide to work on it yourself.