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.
When cleaning anything including baby diapers, it is better to sweep off or scrape off as much solid and semisolid material as you can first. (1) Less cleaning fluid or detergent needs to be used, (2) Less of a mushy mess which in turn means that bleaches or disinfectants will be more effective.