How can you delete save in Pokemon platinum?

All you need to do is start a new game, and then save it. The game will warn you that there is other data on the game card and if you click yes it will wipe the card clean, letting you start a new game.

OR

Go to where it says Press Start and if you want to delete everything you saved, pres Up+Select+B. it will erase everything, it asked you if you want to or not.. so make up your mind over there