Most bikes with gears have two or three big ones up front. Located by your pedals, these are most commonly called chain rings. And you actually won't use them very much at all. Especially when you first get started, you should make a point not to worry about shifting them your front gears at all. Most experienced cyclists will pick one and stay in it probably 90% of the time, and you'll be just fine in picking one and sticking with it.
This is because the big gears in front are for making major shifts in the overall range of your gears. For instance, the smallest chain ring up front is going to give you the easiest pedaling. So if you anticipate a lot of climbing, you'll probably want to operate primarily using the small chain ring in the front, and again, doing most of the actual shifting in your rear gears. If you've got lots of flat riding or down hills, the larger chain ring in front will serve you better. It will allow you to go faster by providing higher gears for you to use, when the bike is already moving fast enough that you can keep the pedals going without the type of exertion it would require if you were going slower or climbing.
Again, the basic premise is to pick one and generally stay there. Youre adjustments should be continuous, smaller shifting through the gears in the back. You'll only shift the between the front rings if the range of gearing in the back, where most of your shifting takes place, is not allowing you to do what you need to do to either climb easily or pedal fast enough on the downhills to keep driving the pedals.