The first thing you should do after buying a winch is to familiarize yourself with the winch and it's components. You should try using it before you get on the trail.
An electric winch is powered by the ATV's battery. The winch motor turns the drum and winds up the wire rope. A good winch will come with a remote control that will allow you to stand clear of the rope while operating the winch.
You should calculate the maximum pulling capacity you will need from the winch. To determine the weight rating of the winch you will need, calculate the gross weight of your ATV and multiply by 1.5. It is important that you do not exceed the rating of your winch or the rope.