After the third Gym there was a house right outside the gym. Inside the girl in red will give you a SquirtBottle to sprinkle on the tree. Sudowoodo doesn't like water, so it'll wake up and fight you. Remember there's only one, so be ready to catch it.

Next head directly to the right and talk to the fat guy that you come in contact with. He'll give you TM08 which is called Rock Smash. This move will allow you to break certain rocks later in the game so don't lose it.

After clearing out Sudowoodo this task will be simple. There's a guy wanting to deliver a Spearow with Mail attached to a guy on Route 31. The guy with the Spearow is in the Route Change gate right after Goldenrod City.

After getting TM 50, you can go back to the guy in the gate, and he'll give you a PP Up.