Pokémon Diamond and Pearl/Twinleaf Town

Twinleaf Town (フタバタウン, Futaba Taun, Futaba Town in original Japanese) is the town in which you and your rival, reside and from which you begin your journey across the region of Sinnoh.

Go down the stairs of your house. Walk outside and there, you will see 4 houses. Your house is the bottom right. Your rival's house is the top left. Go right there. He will interrupt you and will be about to leave when you are near his door. He will forget something and return to his house. Enter it and go up to the second floor.

Talk to him and he will run off again. Go down the stairs and out. Walk northward enter Route 201. Your rival is waiting for you there. He will talk to you and follow you then. So move west and then north near a lake. Pick a pokemon when you reach here.