A Watering Can.
The Watering Can is what you begin with in Welcome To Farmtown!
The Watering Can can also be purchased from Pitchfork Jeb's Farmer Supply, if you should lose it. In order to use it, equip it, and hold the left mouse button to water your crops. The Watering Can has a bar just above it that shows how much water is left in it, as you water this level will drop. In order to refill it, simply go next to your Well, and press the use button.