There are two main types of ATVers and what you need to wear when you ride is a little different depending on what you use your ATV for.
If you use your ATV for hauling items and helping make other chores easier then you are primarily a working rider. If you use your ATV on the forest trails and various dirt tracks, then you're a recreational rider. Whether you're a working or recreational rider, you need to make sure that you wear a DOT approved helmet and goggles to keep dirt and debris out of your eyes.
If you're a working rider, you are going to need more freedom of movement to move items that you are using your ATV to haul. You will need a heavy shirt and heavy weight pants to protect your arms and legs from debris that gets roosted in the air naturally when riding your ATV. Boots to protect your ankle are also a good idea as are gloves.
If you're a recreational rider, your gear needed for riding your ATV will be a little bit different. You will need to wear elbow and knee pads and if you are going to be riding your ATV on a track, wear a chest protector. The force with which a rock can get roosted in the air and hit your chest can really hurt if you aren't properly protected.
Additionally, get a good pair of boots that cover your shins and a pair of gloves that allow for good grip. Many of the gloves designed for riding ATVs now come precurved for a better grip.
Now that you know the basics in gear that you need for the type of ATVing you plan on doing check out the huge selection Raceway offers on these items and best of all shipping is always free. Ride safely!