Instructions
Catch an Oddish or a Gloom
Head to Route 48 in "Pokemon HeartGold." If you're having trouble finding it, check the very left side of your world map.
Wander around in the wild grass until you find either an Oddish or a Gloom.
Attack the Pokemon with one of your own until its health is low, then throw a Pokeball. If successful, you will now have an Oddish or a Gloom in your Pokemon collection. If you caught an Oddish, you'll have to battle with it until it reaches Level 21 and evolves into a Gloom.
Compete in the Pokeathlon
Head to the National Park, which is just east of Olivine City on your world map.
Enter the "Pokeathlon Dome," located on the west side of the National Park.
Speak to someone at the main desk to enter the competition. You will get to choose from a short list of possible activities. For every activity you participate in, you will earn points.
Earn 2500 points, wait until a Thursday in the game, then exchange your points for a "Leaf Stone."
Use the "Leaf Stone" on your Gloom to evolve it into a Vileplume.