Think of your bike as having 3 gear ranges corresponding to the 3 front chainrings. For most riding simply use the middle chainring. When you come to the base of an Oh-my-God hill, shift into the smallest chainring and you'll pedal up the hill with dignity. Save the big ring for downhills and the two days a year that you get a tail wind.

Within the 3 gear ranges, use the rear shifter for fine tuneing. If you think that it's too hard to pedal, shift into an easier gear. If you think that your feet are spinning too fast, shift into a harder gear.

That's all that you really need to know.