Joanne If there is a standing pilot on the boiler. Put the gas valve to the pilot setting and push in on the valve while holding a long match or BBQ lighter @ the pilot assembly that is behind the small access port at the top of the gas train assembly. Dont hold the pilot knob doun without having a flane @ the gas outlet of the pilot assembly or gas will build up and could blow out of the port and hold the gas valve in for about a minute or two and let it go it should stay light when you let it go .If it does not sta lit and it IS a standing pilot there- is a thermo-coupling that may be bad If the pilot does not stay lit.
If there is an electronic ignition rather than a standing pilot then you do not need to light it will light itself if the electronic ignition is bad or if the flame sensor is dirty. If you remove the pilot assembly from the gas train and clean the flame sensor rod with a piece of .0000 steel wool it could help. If not you may want to call a licensed pipe-pitter ( OR plumber depending where you live and the local codes).
If you don't thing you could do it yourself then call a licensed pipi-fitter or plumber. (Gas only has to leak once for there to be a problem so please do call someone licensed if you don't think you could do it your self.), http://www.weil-mclain.com/en/weil-mclain/products-services/
Here is a link to the manufacturer you should be ale to fing your equipment and the install instructions, if you look around .You can download a PDF and print it
Good luck and stay warm