If there are numbers to indicate the gears, this is what you will find:
L - R
1 - 1 Easiest gear to pedal in or low gear. (Small front ring, large rear cog.)
2 - 4 Middle gear ratio. (Middle front ring, middle rear cog.)
3 - 7 Hardest gear to pedal or high gear. (Large front ring, small rear cog.)

In it's simplest terms, use your low gears when you're riding slowly and high gears once you are up to speed (to remember: low gears = low speed -- hi gears = hi speed).
However, unless you are doing some seriously technical riding, you will probably never even go to your easiest gear. Start out in 2 - 4 then progress to 2 - 5, then to 2 - 6.

As you get stronger, you might begin to just keep the left shifter at 3 and only drop to 2 when you're on a hill.

This may all sound like Greek, right now, but as you get used to using your gears, it will start to make sense.