The hoe is used to till fields so you can plant crops. It can only be used on the field of your farm. It can also be used on tilled land to untill it, or used to remove planted seeds from the field (this destroys the seeds).
The can is used to water crops so that they can grow. It only works on tilled fields, and will only add to a crops growth if the crop is planted first.
The scythe is used to cut down weeds which grow throughout your field. It can also be used to cut down crops that have sprouted, removing them from the game.
The fishing rod is used to fish off your dock on the west side of your farm. It can only be used there. Once fishing has started press space again to reel in your fishing line.