If you want your hair to look great, you need to know how to cut bangs so they look appropriate for your hairstyle. Whatever you do, don't cut them too short. You can always cut a bit more, but you can't put hair back once it's cut!
The trick to getting good bangs is to use facial features to measure where you cut them and use as a guide. The large bone on the bridge of your nose is where your hairdresser typically measure's them from, especially if you want your bangs to fall just above your eyebrows. If you want longer bangs, cut bangs using the apple of your cheek bones as a measuring point.
Bangs can be cut either wet or dry. Because wet hair shrinks after it's been cut, always measure longer than what you really want. That's why using the bridge of your nose as a measuring point works so well. Hairdressers (most of the time) cut hair when it's wet. If you want to cut bangs while your hair is dry and you have curly hair, you may want to straighten them before you cut them. Always cut curly hair longer than what you think! You can always cut more later if they're cut too long!
Hair usually shrinks about 1/4" - 1/2" when it's dry. The most important thing to remember is that you must use the same pressure when you hold your hair down to make the cut(s).
Take baby steps if you are unsure about the length. Better to cut too little than too much! If you are too nervous, don't do it or get your mom, neighbor, or a girlfriend to help!