I find I get the best results when shooting in manual exposure and manual focus modes. Auto focusing in low light can be very difficult for many cameras and you’ll end up missing a lot of shots.
If you use this method, you should end up with a pretty good shot of the fireworks. You'll get better with practice, so take a lot of pictures and check out how they turned out after each one. This will give you a great idea whether or not you need to adjust any settings to improve the quality of your pictures.