Mt. Moon!

Feeling like you could take on the world, you head east out of Pewter City.

Brock's crony on the path notices the new body art you're sporting and lets you pass with a sheepish grin.

After a few minutes of walking, you find yourself on Route 3. It's a winding road through a canyon, filled with Trainers and patches of tall grass. The route terminates at the mouth of Mt. Moon, gateway to Cerulean City.

You could battle more Trainers, in the hopes of strengthening your team, or you could attempt to catch some new Pokémon.