You aren't required to register your ATV in the same way you are for a typical highway vehicle. If you're going to operate your ATV solely on private property and not on public lands of any sort—including leased farmland—then you don't need to do anything. However, if you plan on using your ATV on a trail, in a park, along a ditch, or pretty much anywhere else, you need to buy an off-highway decal every year. The decal program is administered by the Parks and Wildlife Department, and fees are used to help maintain trails and promote ATV safety. Many businesses sell the decals throughout the state. Individual counties may require additional registration for ATVs. Check with the county offices where you live to confirm any additional ATV requirements.
Check out these Kerrville, TX ATV/OHV resources: