After going south through Route 17, you reach the next route, Route 18, on your way to Fuchsia City.
Explore the Route
Go southwest from the north entrance of Route 18 and check in the flowers to find a hidden set of 3 Silver Razz Berries.
Go east into the gatehouse, and go east through it to the eastern part of Route 18.
Go southeast and you can battle Bird Keeper Wilton, who has a level 39 Fearow and a level 39 Dodrio. You get 2340 Pokédollars and 3 Poké Balls for winning.
In the grass to the south, you can encounter Rattata, Raticate, Doduo, Dodrio, Pidgey, and others.
Southwest of Bird Keeper Wilton, you can battle Bird Keeper Ramiro, who has a level 39 Pidgeot. You get 2340 Pokédollars and 3 Poké Balls for winning.
Northwest of Ramiro, you can battle Bird Keeper Jacob, who has three level 39 Farfetch'd. You get 2340 Pokédollars and 3 Poké Balls for winning.
To the left of Jacob, there is an Elixir.
Go east to reach Fuchsia City.
Your next destination is Route 12.