Most bikes with gears have between five and nine gears in the back. We are talking about these back gears first because they are the most important to you and where most of your shifting takes place. The shifter for your back gears is usually at your right hand. Get in the habit of using these first.
The shifter on the other side changes the front chain rings. Those are for major shifting that doesn't happen as frequently, but the rear sprockets are what you use almost continuously for small adjustments as you go. It would not be uncommon for you to make several shifts over a couple hundred yards as the terrain goes up and down.
In the back, the biggest sprocket, the one closest to the inside of your wheel, produces your easiest pedaling. The smallest sprocket on your rear wheel, the outermost one, allows you to go the fastest, but is not going to be easy to pedal in unless you're already moving pretty good.
What you're ultimately trying to get the feel for -- and what will become second nature for quickly with just a little practice -- is to shift when you sense that your pedaling is becoming easier or more difficult as it happens, so that you maintain yourself in that perfect and comfortable pedaling spot.
Picture this: the pedaling starts to get just a bit harder because of a small rise in the path and you automatically flip it into an easier gear to maintain your cadence, or pedaling rhythm. Or, the road starts to flatten out and go downhill in front of you and your speed increases, so you flip quickly into a higher gear, and allow yourself to go even faster with the same amout of exertion.
When you notice that you're starting to do this without even thinking is when you'll realize that you've gotten the hang of shifting.
