I shoot fireworks every year - here's the magic recepie....
1. Tripod and cable release. This is really super important. If you don't have a cable, you can just press the button, but they won't be sharp. Or use the self timer in the shortest duration, but you wont' have any control over when to open or close the shutter.
2. ISO 100 is fine, the higher ISO you go, the more noise you'll have in the sky, but the more you'll pick up background glows also, which can be cool.
3. Start around f/ 5.6 (manual mode of course, manual Fstop, and Bulb shutter)
4. MANUAL focus your lens to infinity, then back it off just a hair. Most lenses will actually focus past infinity a bit leaving your shots out of focus. You can test this in daylight - focus on something really far away, then look at your scale and see how close to the focus stop it actually is. Try to manually set it right there during the main event.
5. Turn on your highlight blink-ey preview. Where the pic blinks the blown out areas. Most people when shooting fireworks will blow them out - white streaks instead of pretty purples and greens. That's the real key - catch the color!
6. Shoot in Raw if you have the card space - more margin for error. If you dial it in, then large HQ jpeg should be fine.
When the action starts.....
7. Shoot a couple, then check your preview. If the streamers are blinking they're blown out - stop down a bit. Look at your histogram. Should have a few touching the right side, but not many.
8. Dial your ISO up and down to catch the right amount of background glow. The real beauty of fireworks is to catch some of the natural scenery around the fireworks, not just the pops themselves.
Getting good fireworks takes a bit of feness with the camera - try to dial in your settings good as early in the show as possible, then shoot away. Hey, if you blow it, you've got to wait a whole nother year!