Petalburg Woods is home to many bug type Pokemon, so bring some flying type pokémon, like Taillow or Wingull. Here is also a good place to level your Torchic, so that it becomes a Combusken before the first gym. Bring some potions, as the forest is full of pokémon. If you have not chosen Treecko as your starter, it is recommended to catch a Shroomish, as Rustboro City's gym is weak to Grass type attacks. Slakoth also appears in the grass patches, and can become an extremely valuable member of your team if you choose to evolve it into a Slaking at level 36.
A Team Grunt Attacks!
Eventually, you will see a man who is looking for a Shroomish. After he talks to you, a Team Grunt appears and attacks him. You will have to rescue the man by defeating the guy attacking him.
Come back after having obtained the HM Cut to access the rest of the forest. A girl will give you a Miracle Seed, an item that when held by a Pokémon, increases the power of grass type moves - considering the sheer quantity of water routes and Pokémon in these games, this will come in handy. You can also find a Great Ball and an X Attack, as well as two Tiny Mushrooms hidden in the area.
This is the second Grass Pokémon that you'll find in the game, reminiscent of Oddish and Bellsprout from Firered and Leafgreen and the Red and Blue Versions, but this guy takes its roots more from Oddish. It starts off with Absorb and learns Tackle at level 4. However, when it Evolves at level 23 into a Breloom it becomes a Grass/Fighter Pokemon. Breloom learns good fighting moves later on. So it is a better middle to late game pokemon than an early game Pokemon. You may however, want to keep it on your team, if you don't have a Treecko as your starter. This is also the only area in the game where you can catch one.
This is another Normal Pokemon that you'll find only here in Petalburg Woods, it also only moves every other turn and doesn't learn any good Normal moves. It however can learn HM01 Cut, HM04 Strength, and HM06 Rock Smash so you may want to use it as an HM pokemon, or alternatively evolve it into a Slaking and use it as an offensive wall. But for any Normal pokemon right now, go with Zigzagoon instead.