After you go through Viridian Forest and go north through Route 2, you meet your rival in Pewter City.

Meet Your Rival

Your rival greets you as you enter Pewter City and gives you five Potions. Your rival says you should challenge the Pewter City Gym Leader.

Explore the City

In the Pokémon Center, you can buy one Pewter Crunchies per day from the guy on the left. You can use it to heal all status conditions of a pokémon.

If you go past the grey house to the left of the Pokémon Center, you can go north, then northeast, to find an X Defense near the Pokémon Center.

If you try to use the east exit, a kid there will take you to the Pewter City Gym.

In the northwest part of town, if you agree to watch the Slowpoke for a while, you get a Big Pearl in return. Sell the Big Pearl at the Poké Mart to get 4000 Pokédollars.

You can go into the museum in the northwest part of town for 50 Pokédollars, but there isn't much to do there yet, so you might want to save your money instead.

If you are ready to move on, make sure that you have at least one Grass or Water pokémon (try catching Bulbasaur in Viridian Forest), then go into the Pokémon Gym in the west part of Pewter City.