Hehe! 18's not bad. I have 21, I'm usually using 2 or 3 gears. Chances are, you'll have 6 small cogs at the back and 3 at the front, multiplying together to make 18.
What I usually do with mine is keep it on the largest cog at the front, probably labelled 3 on the left handlebar. On the right, I usually change between 3, 4, 5, 6, or even 7 if I'm going fast.
What keeping it on the largest cog at the front does is enables you to make big changes with one change on the right hand.
The lower the gear (you can find your gear by multiplying left handlebar by right handlebar), the more times you feet will go round per turn of the wheel, thus enabling you to go up hills, and to go fast along the flat.