Use a cable release or wireless remote to trigger the shutter if you have one.
Turn on Long Exposure Noise Reduction.
Shoot the highest quality file you can. NEF is ideal.
Set the camera to a low ISO, such as 200.
A good starting point for aperture is f/11.
Instead of choosing a shutter speed, set the camera to Bulb (B) which allows you to keep the shutter open as long as you want. Expose for the entire fireworks burst. You can even keep the shutter open for multiple bursts.
Turn off the autofocus, otherwise it might have difficulty locking onto focus. Manually focus your lens at infinity.