Your helmet is mandatory and must be approved by the Department of Transportation (DOT). If your helmet is old or has ever been dropped, then it is time for a new one. A full helmet or dust mask is more comfortable when the weather is hot and dusty.
Other recommended equipment includes goggles, gloves, and boots.
As for “appropriate” clothing, well, that depends on the weather and the route. Some people wear running shoes, shorts and a T-shirt but for the most part that attire is unsuitable.
Dress to protect yourself from branches, stones, dust, bugs, wind, sun, rain; from just simply being outdoors.
Dress comfortably because you will likely be straddling a bike for hours.
Dress in layers for the weather and the terrain — add a few items when you’re cold or its raining, take off a few when you’re hot.
Riding an ATV in single digit temperatures becomes extremely cold, similar to riding a snowmobile. Dress accordingly.
Also it is good to have extra fuel, drinking water, snacks, rain gear, bug spray, basic repair tools, band-aids, compass, map and cell phone on hand. Note that there may not be cell service in all areas.
ATVing is a lot of fun. You get to go to places that you have never been before, travelling through some very beautiful areas. Enjoy your time in the great outdoors. Stay safe and have a good ride!