First off I am by no means an expert but since this topic has come up previously, here is my take on the basics of what a beginner needs:
1. The Quad, buy used or new both have their pro's and con's. Ask people what they ride and for their reccomendations. Ask the dealer. After you get it take the safety course which is usually free if you buy a new quad from the dealer.
2. A helmet. Buy the best you can afford. Nuf said.
3. Gloves.
4. Sturdy Boots. Ideally you would want true cycle boots, but for just starting off good stiff boots will work, but you will want to invest later into a good pair of riding boots.
5. Repair manual for the quad.
6. Basic tools, sockets, screwdrivers, pliers, etc. Add to your set as needed. Try not to skimp on tools as they will last a lifetime for good ones.
7. Gas can to haul extra fuel.
8. This is probably the most important and expensive. If you don't live near a riding area, you will need a vehicle(truck to haul your bike in the bed) or car/truck/van and trailer to get your bike to the riding area. Otherwise having a quad and no way to get it to the riding spot, makes the quad worthless.
9. Tiedowns to hold the bike on the transporting vehicle/trailer.
10. Insurance on the bike. Optional but strongly reccomended.
These are the basics.
Add the mods and clothes and other protective gear as you can afford them although I personally would reccomend the protective gear before the mods.
If any one has anything else to add please do, but remember this is for the complete newcomer to the sport.
Wheatchex