Sandgem Town is a town found in Pokémon Diamond and Pearl. It is the second town you come to in the game, the first being Twinleaf Town. Sandgem Town has two houses, one of them being the character's other rival's house. It has the Pokemon Lab which is run by Professor Rowan, a Pokemon Center and a Pokemart. This town will be very useful after you beat the Elite Four and get a National Dex. Once you get the National Dex you can go to the house directly below the Pokemon Center. This is Dawn/Lucas's house. Talk to their little sister and she will tell you where a Pokemon Swarm is.
Once you reach Sandgem Town, Prof. Rowan's assistant will greet you and take you inside the lab. Just as you are going to enter the lab, your friend came out and hit you. Your friend will say that Prof. Rowan isn't as scary as he thought. He then sped off. Then head inside and talk to Prof. Rowan. Prof. Rowan will ask you to complete the encyclopedia Pokédex and if you refuse. He will reply "..." several times until you agree. Prof. Rowan's kid will be holding a Pokémon that you and your friend did not choose. If you and your rival choose Turtwig and Chimchar, he/she will choose Piplup.
Head outside the lab and the Prof. Rowan's kid will show you around the town. He/She will show you the Pokémon Center and the Pokémon Mart. Then she tell you to inform your Mom that you are going out for an adventure to complete the Pokédex. Then he/she will run away to Route 202. You must head back to Twinleaf Town to tell your Mom because the kid don't allow you to pass Route 202.
If there is a Pokémon Swarm, the Prof. Rowan's kid's sister knows it. Head to Dawn/Lucas's house and ask her. She will tell you the Pokémon and the locations. You need to defeat the Elite Four first.