Hi. I did a 4 month placement in Fiji in 1994. I would recommend getting out of Nadi as soon as possible. VitiLevu is nice in the south but it is very touristy.
Taveuni Island is worth the trip. There is one hotel called the Garden Island Hotel and the diving is great there. Try the Rainbow. About 10km away from the hotel is camping right on the beach. If you are a camper I would highly recommend it. There are also great hikes that you can do at Taveuni. It is where the Blue Lagoon was filmed. Very close to the hotel are other short hikes. Ask about the natural water slide. Someone will take you.
Lanua Levu is also worth while. It is a quite island with little tourism, nicer lodges and less expensive. Ferries are cheap, but you can also fly. If you are pressed for time you could stay in the Nadi area. Resorts are approx. 100.00 per night. The resorts are very nice but far from the reality of Fijian life style.
If you wanted to stay in a village over night, bring a bunch of Cava root to the chief and you will be able to stay. They are very friendly. Cava root can be purchased just about every where. Try the markets or just ask someone.
May to September is their Monsoon season. The Nadi area and towards Suva may be wet but the Lautoka area (east) is fairly dry. After September it gets hot!
Swimming in the ocean isn't always possible, unless you take a boat out. There is a lot of coral. Always wear shoes. At about 5pm the woman spear for octopus. Ask if you can join them.
If you want to buy a book the Lonely Planet is pretty good but I used it only as a back up. Talk with the others while you are there. Well, I hope this helps you.
Very difficult to give you an answer in a nutshell, but try Lonely Planet's 'Travel Survival Kit - Fiji'. It should be available in any bookstore that stocks LP titles. We used it extensively. Prices etc since when we were there last will have gone up so I can't help you there. Suffice to say, it is a fantastic place :))
I am interested in going to Fiji for several months . Can you suggest what would be the best available address, literature or info on the subject? Please, could you also suggest modestly priced hotels, food, etc. while in Fiji , and how to make reservations. Thank you very much for your time.