Turtwig, Chimchar, and Piplup.
you cant in platinum but you could trade from Diamond and Pearl or another Platinum.
the three starter Pokemon are the same in platinum as diamond and pearl. They are chimchar-fire, piplup-water, and turtwig-grass.
still the same Pokemon as d/p chimchar, piplup, and turtwig
Just follow the storyline at the start and you will meet the professor and he will let you choose a starter. Piplup:Water Turtwig:Grass Chimchar:Fire
trade with friends and/or wifi trading
try to catch your legendary. It varies on which legendary depending on what starter you chose.
starter Pokemon
Chimchar, piplup, and turtwig.
Pick him as your starter
Piplup
You can get another starter if you get all the sinnoh pokemon. Proffeser rowan will ofer a johto starter.
you get piplup as a starter in Pokemon diamond/pearl/platinum