First off, congrats on your first 5k! This is a big milestone, and the first race is always extremely exciting - albeit a bit nerve wrecking at the same time! And you are right for wanting to think about how to best prepare yourself.

The first thing to do is consider weather and temperatures, as well as where you are racing, when (what time of year), and what time your run starts. You will want to dress for the weather, and that means keeping your eyes on inclement weather possibilities that might creep up in the forecast. If temperatures are expected to be high, and humidity might be a possibility, then dress as cool as possible. In fact, even if temperatures SEEM cooler, dress for heat. It is MUCH better to under-dress than overdress (unless, of course, you are running in conditions in which hypothermia or other extremes are possible.) It is far more likely that you will warm up after the first mile or so, and then you will be comfortable and able to push harder. I advise going with shorts, a tank top and loose fitting top. And it is even better if all your clothes are moisture wicking.

Running a good race also comes down to a good pair of shoes. Make sure you have actually worn and run in your shoes before though - the last thing you want to do is try a new pair of shoes the day of your race and possibly end up with feet full of blisters. Get good socks too - thicker socks provide more cushion and a moisture-wicking pair of thick socks will keep your feet dry and comfortable, while not overheating them (even if the sock is thick).

Because you are only starting with a 5k, fueling and carrying along water bottles probably won't be necessary. But if you continue to race and want to try out longer distances in the future, then consider investing in a running belt, which can hold your fuel, water bottles, and other necessities. And you might want an armband or something to hold a music device, if you are set on running to music.