You can not get a Turtwig egg in Pokemon Platinum. But if you breed a Turtwig with a Ditto, then it is possible.
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starter Pokemon
at the begining of the game the profeser will give you three Pokemon and you have to pick one and those three Pokemon are turtwig chimchar and piplup and if you want turtwig pick turtwig.
Turtwig is a starter Pokemon in Pear, Diamond and Platinum or you can obtain through trade
18 then 32
you get it as a starter in diamond, pearl and platinum