This will allow you to hold down the button and water a greater area of plants with every upgrade. Prioritise your watering can, in the beginning. Head to Clint the Blacksmith to exchange money along with bars of copper, iron, gold, and iridium to incrementally upgrade your tools.
Upgrading your tools will let you break all kinds of stuff you couldn't before! You've probably found rocks you can't break, wood you can't cut, and watering your plants takes forever? The good news is this doesn't have to last forever.