When shooting fireworks you need to have a tripod or a very steady place to put your camera. What i've done in the past is set up my camera on a tripod and used a remote to snap my shutter. It is the best way. You will also need to open your aperture fairly wide id say F5.6 or lower. Because you are trying to take a photograph of a quick moving subject at night, slowing the shutter too much may just blur your fireworks. Unless thats what your going after. You need to adjust the shutter and time it with how long the lights hang in the air. Dont be afraid of going into the higher iso as well. Try not to go too high as the higher you go the more noise or grain your going to see.
Here is an example of a fireworks shot I did this summer.