what is the fastest way to lvl up a starter pokemon on platinum

[Psyduck]

what is the fastest way to level up a starter pokemon on pokemon platinum

[Zidarose]

Aside from Rare Candies, I would recommend trying one of the few areas you have another trainer with you, like when you first go into Eterna Forest, the Wayward Cave or Stark Mountain.

You could also read this guide to get more suggestions on how to level up Pokemon fast:
Pokemon Platinum Leveling Guide