erik911sd saidWhen people have told me that shifting is confusing and that they never know what gear to be in, I tell them it's easy: If your legs are spinning fast and you're going nowhere, shift to a harder gear. If you're straining with all your might to turn the crank, shift to an easier gear. Most of your shifting will be in the back through the 7 cogs, and then shifting up or down on the front through the chain rings depending on the terrain. You've got 21 options, but you'll figure them out pretty easily once you get out there.
I concur. The purpose of the gears is to give you a pedal rate that is comfortable for your abilities, and is consistent. The term for your pedaling rate is "cadence", that is, the rotational speed of your pedal crank, how fast your feet are going up and down.
I like a cadence that's between 60 and 70 rotations per minute, regardless of my forward speed. The gears allow you to maintain this steady pedaling cadence, even as your forward speed varies, up inclines, on flat terrain, and against or with the wind.
Experienced riders just sense their cadence rate, but you can also get an electronic speedometer that displays both forward speed in MPH and distance traveled, as well as your pedaling cadence, using an additional sensor near the crank. A bike shop can assist you.
Cadence tends to be more critical for road bikes going long distances, than with mountain bikes. Later you may wish to add another bicycle to your "stable" that's a road bike. Each kind of bike is suited to a different job.