Now, it's time to buy plenty of poké balls and potions.etc. Start exploring the city. There is a mart, Poké Center, a locked gym, etc. Once you're ready, head North-West to Viridian Forest. Follow the path, catching different pokémon and fighting trainers. You can't run from trainers or catch their pokémon in poké balls. At the end of the path there is a house. Go inside and out the other and you will end up in Pewter City.
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Pewter City Walkthrough