Here's how:
* Find Caterpie: Caterpie is the pre-evolution of Butterfree, and it's fairly common in Viridian Forest. You'll likely find them throughout the forest, especially in the grassy areas.
* Train Caterpie: Caterpie evolves into Metapod at level 7, and Metapod evolves into Butterfree at level 10. So, you'll need to catch a Caterpie and level it up to get your Butterfree.
Tip: Viridian Forest is also home to other Pokémon that can be useful in the early game, so it's a great spot to catch some early-game Pokemon and level them up.