Pokémon Diamond and Pearl/Route 202

Events
Enter Route 202 and into the first grass you see. Professor Rowan’s assistant (Lucas if you’re playing a girl, Dawn if you’re playing a boy), will call you over and show you how to catch Pokémon. It’s helpful to lower the foe’s HP into the yellow or red before throwing the ball. The assistant will then bestow upon you 5 Pokéballs.

Wild Pokémon
Pokémon found in the tall grass:

Trainers
Go east past the grass bush and north through another bush. A trainer stands in front of some trees at the far end.

Head south from Tristan and stop when you’re facing a column of trees. Take two steps down and travel east you’ll unavoidably come into a trainer’s view.

Head east and north through tall grass. At the end is a trainer.

Head east past the main track and into the grassy bush. You’ll find a Potion. Leave the bush and walk back onto the main path. Head northwards to Jubilife City.