The egg you receive will hatch into Togepi after you carry it around for a certain number of steps. If you start carrying right away, it'll probably hatch around the time you reach Ilex Forest.
Chat with our AI personalities
You go to prof. Elm and talk to him (you have to have togepi in your party)
When it likes you. This is one of the Pokemon that evolve through friendship.
A Togepi hatches from the egg.
You can't get Togepi in Pokemon Yellow. Togepi does not exist in that Pokemon generation. (:
how to get a togepi in Pokemon platinum