After you meet your Dad, Norman, in Petalburg City, and help Wally catch a pokémon, you continue on to Route 104.
Explore the Route
In Route 104, you can find Zigzagoon, Wurmple, Taillow, and Wingull.
To the south, on the beach, you encounter Youngster Billy, who has Seedot and Taillow.
North of Billy, you can talk to the sailor, Mr. Briney, and his Wingull, Peeko.
A bit to the north, there is a boy who needs a Potion. You can give him one if you feel like it.
North of him, there is an Oran Berry tree and a Pecha Berry tree.
West of there, you encounter Rich Boy Winston. He has a Zigzagoon.
Go north from Rich Boy Winston to enter the Petalburg Woods.