I'm stating the obvious but you're going to be using a long exposure. So you need a tripod, as much as I loath tripods, they are useful in this case. Any slight movement will result in a blurry photo so a tripod will keep your camera steady.
BULB Mode & Shutter Release Cable
You need to set the shutter speed to Bulb mode. This will capture a firework trailing and or bursting in the sky. This is what you're going to be using the long exposure for! It's really tricky though, to learn where a firework is going to explode and where to point your camera. Using BULB mode will get around this problem. It will hold the shutter open for as long as your finger is pressed down on the button - or in this case, your remote shutter release cable thingy. You'll likely need that, it reduces the need to touch the camera - less blur. So, press down when the firework goes up and release when it explodes in the sky. If you can double expose or lock the shutter open and cover the lens with an object, you can get some neat effects and multiple bursts in the shot from successive fireworks. Be careful, fireworks are bright and can quickly overdevelop areas or all of your shot.
Firework displays are bright - despite it being nighttime. Therefore a large aperture may cause overexposure in your photograph. Small apertures will control the brightness and bring out the best in your colours. There's not set rule but using an Aperture of around F8 if far away and F16 if nearer will suffice. Remember when you change Aperture, focal length changes too!
Keep your ISO low - around about 100 more or less, should keep it low enough to reduce the grain and give you a cleaner image.
You can zoom in, zoom out, shoot landscape or portrait - whatever - when it comes to fireworks. So get creative and learn what's best for you so that you can achieve the best results possible! Close-ups are good and can be amazing, but you'll also want to take some wider shots of surrounding elements to help frame the image and give it perspective.
You'll kick yourself if you get home and realise that every shot you took has completely failed. Really. I've done it before. Check your results. Make sure you aren't flopping on every shot. Remember, you'll have some duds, that's why we take so many shots!
Even though it is normally really dark anywhere you may be shooting fireworks, if you can, still use a lens hood to keep the risk light from the areas around you to a minimum.