I need to evolve my togepi!
In the National park, if you go behind the fence up at the top of the park. Then you can follow the fencing until you see an item and it will be a soothe bell. However, you don't absoulutely need it to evolve Togepi.
You can get the Soothe Bell as soon as you reach Goldenrod City. Head north from there and go all the way up to the National Park, then go to the northern side of National Park near the NPC playing a game on their Game Boy. You'll see a broken link in the fence there, and you can go behind the fence towards the right to find the Soothe Bell.
(Source: Guide Strats)