Rare, you have to use the Pokeradar on Route 230 to find it.
You get a togepi egg in Pokemon Platinum, not Diamond.
You can only get Togepi in Pokemon Emerald by trading from another game. Togepi is a baby egg Pokemon that prior to Gen 4 was normal type Pokemon.
you need to hatch a togepi in soulsilver or heartgold then trade it to pokemon diamond
You bring it to level 50
Leave it with the nursery
Pal park, trading otherwise immposible. I have platinum, and when you go to eterna city cyntia gives you a togepi egg.
Route 230 with Pokeradar
I am assuming you are talking about togepi. To be honest, I'm not sure if you can get togepi in Diamond without trading from Platinum. That is how I got mine. Even in Platinum, you can only get it by hatching an egg that is given to you. Sorry I didn't really help much, but I will tell you now that you have great taste for wanting one. I traded rotom for one, and I will never regret that. It is now a Lv.100 Togekiss (besides, rotom sucks!).
You must first get Togepi on route 230 using your poke-radar. Then you must get Togepi to evolve into Togetic. Togepi is a happiness evolution, which makes sense as Togepi is a egg Pokemon.
route 230, you will need to use the pokerader.
use the poke-rader on route 230
#175 Togepi Rare-PokeRadar Route 230