None of the DS Pokemon games (Diamond, Pearl, Platinum, HeartGold, SoulSilver, Black, White, Black 2, and White 2) allow you to choose Squirtle as your starter. However, in HeartGold and SoulSilver, Professor Oak will allow you to choose one of the Kanto starters after you've defeated Red on top of Mt. Silver, however that is very late in the game and is considered to be one of the hardest battles in any Pokemon game. That's the only way to obtain Squirtle legitimately using only the DS generation Pokemon games. There's also no way to obtain a Squirtle as soon as you start the game, unless you have two systems and another game with a Squirtle, which you can trade to the other game you want the Squirtle on after you progress a little bit.