If you have the patience then I have a piece of advice on how to have your team awesome before facing Brock.
By this stage you should have six Pokémon including your starter, Treeko, Mudkip, Torchic and other Pokémon from the wild. In Pewter city there is the Pewter city garden. First you will find a sun stone. In here there are a few Pokémon to capture. 
Nosepass: Level 11 with attacks: Harden and Tackle-Earn 169 exp.
Slakoth: levels 11-12 with attacks: Scratch, Encore and Yawn-Earn up to 142 exp.
Swablu: Level 10 with attacks: Peck, Growl and Astonish-Earn 105 exp.
Skitty: Level 10 with attacks: Tackle, Growl, Attract and tail whip-Earn 92 exp.
Taillow: Level 12 with attacks: Peck, Quick attack, Growl and Focus Energy-Earn 101 exp.
Sunkern: Level 9-10 with attacks: Absorb and Growth-Earn 74 exp.
As you battle these Pokémon the exp builds up quite quick as your Pokémon will be about level 10. So before you beat Brock this is a good way to have your Pokémon leveled up to level 19 (Recommended as you need one badge to control level 20 and above Pokémon)
I have done this as before I beat Brock my team consisted of Grovyle level 19, Combusken level 20, Marshtomp level 20, Taillow level 20 and Elekid level 20.
I hope this helps some people so go get training

B1ad3 : dude how did u get all three hoen starters

Talk to people in towns and you'll find them dude

Wooten101 : i think you would want to hear this. 

B1ad3 : Thank you I trained my Pokémon up to level 20 there

Beastmode64 : it's really frowned upon when you post things that doesn't contribute bud.

You're welcome goutham. And I'm sorry for not giving you a more informative answer Wooten101. I just couldn't remember about how to get all Hoenn starters but I remember that I spoke to everyone in towns and in buildings and eventually received all three from different people.

Just wanted to note that there is level 13-14 Nosepass as well.13 and 14 know Rock Throw and level 13 give 200 exp