A well is required to fill up a watering can for farming. Wells in turn require cement to build which may need a larger rucksack than the Bunny Wabbit Backpack to carry (if beginner). Wells, like water towers, can provide water for your community as well. As with other constructions wells require a builder to construct. You will have one builder on completing the initial tutorial tasks successfully.
Once you have the required cement and Wood Planks, start building where you want the well and choose the well from the building options (PC: use mouse wheel/button by default to cycle through the available build types) and begin construction.
Alternatively you can use a well from an existing NPC (non-player character) community to fill the watering can.
A well is an infinite water resource. It will never run out of water, so you do not have to worry about scavenging towns for water. Wells also allow farmers to refill their gardening cans, allowing them to farm. It is recommended to build a well close to a farm so that the crops can be maintained easily.