Choosing a filed is not as simple as calling and booking a date. The best price in town may not be the best field in town. Contact your local field. They can be found under Paint games, or Adventure in your yellow pages. Also paintball publications will have field listing in the back of their magazines.
Extra paintballs are available at the field.
Better markers and facemasks are usually available at an additional cost. Clothes that you don't mind getting sweaty, and that are suitable for the weather conditions on play day. You should bring some snacks, and plenty of water. Also, notify the field owner or manager of any medical conditions, allergies etc. The field should provide everything you need, in the way of safety equipment.