Togepi evolves by raising it's friendship, which means there are many methods one can partake in evolving it.
Here is a table, with all the methods of increasing friendship with your Pokemon.
Of course, the methods marked with 'HGSS' are only available in HGSS, and the methods marked with 'DPPt' you can still do.
The ways you can do it pre-Elite Four are:
- Walking 256 steps (+1)
- Massage from the woman in Veilstone City (+3)
- Using Vitamins (varies)
- Levelling Up (varies)
- Using EV-lowering berries (varies)
Note, there are also multiple methods that can lower your Pokemon's happiness, such as fainting or using medicinal herbs. Make sure to keep Togepi alive and healthy!
Your Pokemon's happiness must reach 220 in order to evolve.
And of course once Togepi evolves into Togetic, you can evolve it into Togekiss with a Shiny Stone. Shiny Stones can be found on Iron Island and Route 228, as well as on Route 210 (exclusively in Platinum).
Hope I helped. :)
Source and above links