1. Read. Learn. Whether it be books about exposure, books about composition, using flash, etc.. The more you learn, the better photographer you will be. And this is the cheapest way to improve your photography.
2. Along with #1, learn to shoot RAW. Shoot RAW and process your photos yourself in Lightroom or Nikon's software. The images will look flat at first. But with some practice, you will get better results than letting the camera process the jpegs. It does add time to your workflow.. you may not want to shoot everything raw. But for your "special" shots, like fireworks, etc.. you definitely want to be able to process yourself.
3. Without breaking the bank, the simple important gear would be a decent tripod (especially if you want fireworks!), some photo processing software (See Lightroom, #2 above). And perhaps an external flash, if you learn to use it properly. (#1 above).
Good luck and enjoy!