Polit Light Won't Light?

Pilot light on my gas boiler in the back bedroom won't light. You open the bottom of the boiler which looks like a big white rectangle box with a little flab at end with a thermostat, ignition push on and i think another red on for the gas. I am holding in the gas on while pressing and releasing the ignition on but nothing is happening i am getting a electric spark but the light is not lighting. I hope people can understand this and maybe give me some help, thanks in Advance.