Instructions
With Totodile as Your Starter
Select Totodile as your starting Pokemon and receive the Pokedex from Professor Oak.
Walk back toward New Bark Town. On your way back you will encounter your Rival right after you leave Cherrygrove City on the road to New Bark Town. He will challenge you to a Pokemon battle and since you chose Totodile, your Rival will choose Chikorita, which is strong against Totodile.
Defeat your Rival's Chikorita using your strongest attacks to obtain the entry on your Pokedex.
With Cyndaquil as Your Starter
Arrange a trade with a friend that has a copy of either "Pokemon HeartGold" or "Pokemon SoulSilver" and chose Cyndaquil as a starter. If you have both games you can trade between your own cartridges using two Nintendo DS systems.
Open your Nintendo DS configuration menu and connect to a Wi-Fi Internet hot spot by following the on-screen instructions for setting up a network connection.
Visit a Pokemon Center after obtaining your first Gym Badge to trade Pokemon with a friend via Wi-Fi Internet connection. To make a trade, visit the Union Room upstairs at the Pokemon Center and talk to the woman behind the counter. Your friend must do the same. Select which Pokemon you would like to trade and confirm your selection to make the trade. Chikorita will automatically be added to your PokeDex.