Confirmation is a special ceremony in the Catholic church when young teens, usually between ages 13 and 16, become adult members of the church. This occasion is often attended by close friends and relatives, so you want to look special while still dressing appropriately. Bear in mind that you may wear a confirmation robe over your outfit, depending on your church.
For Boys
Boys should dress a little more nicely than they would for an average mass. Wear black slacks with a belt and a button-down collared shirt. The shirt doesn't necessarily need to be white, but it should definitely be a solid color. Make sure to comb your hair, tuck in your shirt and wear black dress socks and shoes.
For Young Girls
Confirmands on the younger side of the spectrum may want to go big and lacy. You don't want to veer into flower girl territory, however, so keep it somewhat subdued. Also remember that you could be under a robe, and you'll be in front of the congregation for quite a while, so comfort should be a priority. On the other hand, a sleeved, "tea-length" (mid-calf) simple white dress can be both elegant and timeless. Consider silk or cotton fabrics, depending on how formal you want to look.
For Older Girls
Older teens will probably be more comfortable in something with spaghetti straps, or even strapless. Instead of buying a conservative dress you may never wear again, go ahead and pick out a strapless dress of an appropriate length: at least a few inches below the knee, or even a maxi dress. Then, pair it with a simple white cardigan, shrug, or bolero for the service so that your shoulders are covered.
For Girls Who Don't Want to Wear a Dress
Some girls just don't like dresses. Your parents may panic at the idea of a pantsuit, however, and jeans are definitely out of the question. Consider wearing a simple, a-line, white skirt with no pleats and an unembellished sweater or top. Dresses are extremely traditional, and since it's only one day it may be worth biting the bullet to appease your family. However, a skirt is a nice compromise that can quickly be changed out of after the ceremony (and pictures).