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.
(One morning on WRKO Radio) The tub's not clean! Let's remodel the bathroom.
(One afternoon on WRKO Radio) It's time for an oil change. I'm going to trade in the car for next year's model.