Instructions
Obtain a Chikorita. In the Diamond, Pearl, and Platinum games, you'll need to import a Chikorita from a GBA game through the Pal Park, or trade with someone who has a Chikorita in their HeartGold or SoulSilver game. In HeartGold and SoulSilver, Chikorita is one of the three Pokemon you may choose at the beginning of the game.
Put your new Chikorita in the daycare along with the Pokemon Ditto. Hatch the eggs that the Daycare Man gives you until you find a Chikorita with a nature that increases one defensive stat without lowering another. Good defensive natures to look for are Bold, Impish, Calm and Careful.
Choose the defensive moves you would like your Chikorita tank to have. Two moves should be "Reflect" and "Light Screen" as these will provide an enormous bonus to your Chikorita's defensive capabilities. The third defensive move should either recover HP or status ailments, so choose "Synthesis" for the former or "Aromatherapy" for the latter.
Activate TM 06 to teach your Chikorita the move "Toxic." Use this powerful new move to defeat wild Pokemon, gain experience and move up the levels.
Teach Chikorita the HP-restoring move Synthesis when it attains level 12. Afterwards, teach it the moves Reflect at level 17 and Light Screen at level 31. If you would like to replace Synthesis with Aromatherapy, continue to level Chikorita to 42.