Pewter City is the second city a trainer will encounter. The city's Gym Leader is Brock, a professional-type Pokémon trainer.
In the PokéCenter, an NPC will tell you what and where swarms are going on. Swarms usually include very rare Pokémon and starters, as well as the Mythical Pokemon Shaymin. They change every 10 minutes so make sure to come back every few minutes.
Pewter Gym is a-type Gym with Brock as the Gym Leader. This is also where you get your first badge: the Boulder badge. When you enter the Gym, you must face an apprentice/admirer of the Gym Leader.
There are some orange grass patches at the entrance of the city.