Brock is the first gym leader you encounter. He uses rock-type Pokémon and he resides in Pewter City(Pewter, by the way, is a type of gray). He's a cinch to defeat if you picked Squirtle(make sure he has learned "Bubble" by this point) or Bulbasaur(make sure he has learned "Vine Whip" by this point) since both their types have an advantage against his Pokémon types. But if you picked Charmander in the beginning, you might have to level Charmander up or any Pokémon that you have. But Flying, Bug and Electric Pokémon(they're the types that have probably appeared in Route #1 or Viridian Forest) are weak against Rock so I suggest training Charmander to L15 so that he learns "Leer".

Since Onix's defense is very high, an attack like Scratch can barely touch Onix(grr). So if Charmander learns Leer, it can lower Onix's defense, then go for the kill with Ember/Scratch. Or...you can catch a Caterpie in Viridian Forest(they appear commonly in Blue version, but very rare in Red version...pooey) and level it up to become a Metapod then Butterfree to L12. At that level, Butterfree knows Confusion.

Yellow version has it better, since in that version, you can catch Mankey earlier and it can learn Low Kick at L9(earlier than in R&B). And fighting has an advantage over rock.

When you have bested Brock, he'll give you the Boulder Badge. The Boulder Badge increases your Pokémon's power and it allows you to use HM05, Flash anytime.