cheats

how can i download cheatcodes to my DS? And how do i use them?

Flashcart?

i got a r4i if that is what u mean ;D

Download the cheats database, put it on your flashcart, and then enable/disable the cheats you want.
http://cheats.gbatemp.net/

but for some games (like pokemon platinum) there are no cheats. How do you get those?

The link that TirithRR posted has platinum cheats on it (checked) ... think your gonna have to be more specific on which games.