Instructions
Go to Goldenrod City. Head north and east of the Pokemon Center to find Goldenrod's Flower Shop. Talk to the girl working at the shop and she will give you a squirt bottle.
Head east out of Goldenrod City toward Route 36, and you will come to the intersection where the "strange tree" is blocking your path. Tap "A" and you will be asked if you want to water the "strange tree." When you sprinkle water on it, it will attack, revealing that it is not a tree at all but the Pokemon Sudowoodo.
Battle the Sudowoodo, whittling down its health. Sudowoodo looks like a tree, but it is a Rock-type Pokemon, making it especially weak to Grass and Water attacks. K.O. the Pokemon or catch it when it is weak using a Pokeball.