Turn the little knob from the "pilot" position to the "on" or "run" position.
Slowly turn the large (temperature) knob all the way down to the lowest or "vacation" position. Then slowly turn it up. You will probably hear a click and then the burner should come on. If not there is probably a problem with the entire control assembly.
If you get it to work but the temperature knob has to be in the "very hot" range to get decent performance, then the control parts are showing their age.
Do you get coverage you can count on? From the bag of fertilizer? From your lawn sprinklers?