Pokémon HeartGold and SoulSilver/Violet City

../Violet Gym/
You'll see the Violet City Gym when you enter the city, but you can't complete it until you have gone through the ../Sprout Tower/. It's recommended that you take a detour to ../Route 32/ before you head into the tower to catch a for training. Mareep's electrical attacks will make the flying-types of the gym trainers even easier.

Pokémon Academy
If you talk to the man standing between the Gym and the Mart, and answer "No" to the question he asks you, he will lead you down to his Pokémon Academy to study up on Pokémon types and conditions. In this schoolhouse, he teaches his students some basic Pokémon battling and raising lessons. You can also learn a lot from the writing on the blackboard and the other students. The man to the farthest right of the room will offer you to create a group for your friends to join. This is covered fully in the ../WiFi/ section.

Trade Bellsprout for Onix
You can catch an later, but you'll never get as much use out of one as you will now versus Falkner and his Junior Trainers. Visit the house southwest of the Pokémon Center with an extra in order to trade it for it. If you're looking for a Bellsprout, you can find one on ../Route 32/. As a traded Pokémon, your Onix will go up levels more quickly than a wild one, but you're stuck with the nickname the trainer gave it. Like all trades with computer characters, its starting level will be the same as that of the Pokémon you traded for it.

Onward to ../Sprout Tower/!
After you've replenished your goods at the Poké Mart, stopped by the Pokémon Academy, and traded for an Onix, if you wanted to, head east from the Poké Mart and head north at the bridge to reach the tower. This is a good place to train your Pokémon before the Gym, and it's completion is mandatory in order to proceed.