Petalburg Woods is home to many bug type Pokemon, so bring some flying type pokémon, like Taillow. Bring some potions, as the forest is full of pokémon. If you have not chosen Trecko as your starter, catch a Shroomish. Shroomish has some nice attacks, and can be very useful in the next town. Slakoth also appears in these grasses, but its moveset is very bad, so use Zigzagoon instead. There are many hidden items here, do not forget to look for them.
Eventually, you will see a man getting attacked by a Team Grunt. You will have to rescue the man by defeating the guy attacking him.
This is the first 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. But for any Normal pokemon right now go with Zigzagoon instead of Slakoth.
Slakoth
Taillow
Silcoon
Wurmple
Zigzagoon
Poochyena
Cascoon