Permits are required from the state or county offices. The number of nights stay is limited and permits are checked. Air mattresses are great for increased comfort and you won't get wet if you do experience some rain. If you're staying in the mountains you'll need warmer gear. It can get very cold at higher elevations.
Mosquitoes are abundant, especially in wetter areas.
It's available at health food stores. Rental shops have gear available and discount stores such as KMart and WalMart carry all the equipment you might need. My first trip to Kauai was camping all the way. On the second trip, we scheduled a few nights in a hotel. After one night in the hotel, we went back to the beach.