Instructions
Enter the Viridian Forest and walk left into the tall grass. Head north, then left again and press "A" to find a Poke Ball.
Return to the entrance of the forest and walk east. Continue north up the path to reach an Antidote. There are two Bug Catchers who will challenge you to a battle, both using Bug-type Pokemon ranging from Level 6 to Level 7.
Head left into the tall grass and continue south. Follow the path as it curves north, then south again through two large patches of grass. When you reach the signpost, head east then south to reach a Potion.
Return to the signpost and walk west, then north up to the last patch of tall grass. Another Bug Catcher will battle you with a Level 9 Weedle on his team.
Continue north through the exit to reach Pewter City.