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. Follow him to Route 201. He will say that you should go to the lake to find the Red Gyarados. He will then follow you around, and since you can't walk in tall grass (or go back to Twinleaf Town), you'll have to head left to Lake Verity. A word of caution, save before you go to the entrance if you want to have more than one of the starter Pokémon you choose. This is because if you don't save and you're attacked by the wild Pokémon, and the starter you use in battle is Male, then the eggs it produces will not be baby starter Pokèmon unless it is bred with a Ditto.

Go through the entrance to Lake Verity. You will see a girl (or boy, depending on which gender you chose) and an old man standing and talking at the shore. They will thereafter exit, politely asking you to move aside so they may do so. Your rival notices that they left behind a bag of some kind. He says there's no harm in going into the grass for a second to see what's in the bag. That turns out to be wrong, as you are attacked by a Starly, at level 2.

To fend off the Starly, you are given a choice of three Pokémon: Piplup, Chimchar, and Turtwig. Piplup is a great starter for defensively minded players, as its evolved form, Empoleon, has resistance to 12 of the 17 types. Turtwig, although not especially powerful, is easy to raise. Chimchar is hard to raise, but is a great attacker. Choose whichever one you feel is the best or your favorite, as they will all help you in your journey. You should easily defeat the Starly with your newfound Pokémon.

After you defeat the Starly (and gain some Exp. Points) the young person you saw earlier will come back to fetch the briefcase, and sees that you took a Pokémon each. (S)he says something about it being "bad" and rushes off. You will exit the lakefront and walk up to the girl (Dawn) or boy (Lucas) and the old man (Prof. Rowan). Some dialogue will go back and forth, ending with an invitation back to Prof. Rwoan's lab if you have the chance. You'll get that chance very soon, but for now head back home to your mother and get healed.