you will see hapinny in route 210
I think you use a round stone. I found the evolved version of hapinny in the wild;)
Hapinny
for hapinny to evolve into a chansey
hapinny into chansy
Talk to the mansion owner after you get the national pokedex
Give it an oval stone and level it up.
I always thought it was 7,326 steps.
At the lost tower or at a wild chansey or hapinny. check out: http://www.serebii.net/diamondpearl/items.shtml