North of Lyle is a Paralyz Heal. Grab it and move on. As you encounter a Devon Researcher, he asks you if you've seen a Shroomish. As you're talking, a Team Grunt (Team Magma if playing Ruby, and Team Aqua if playing Sapphire or Emerald) appears and asks him to hand over some papers. The Researcher hides behind you, and you'll have to battle the Grunt.
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 start off with Absorb and learn Tackle at level 4. However, when it Evolves at level 23 into a Breloom it become a Grass/Fighter Pokemon. Breloom learn good fighting moves later on. So it 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.The is also the only areas 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 move every other turn and don't learn any good Normal moves. It however can learn HM01 Cut, HM04 Strength, and HM06 Rock Smash so you make want to use it as a HM pokemon. But for any Normal pokemon right now go with Zigzagoon instead of Slakoth.
Items
Paralyz Heal
Ether
Poké Ball (hidden in grass to the right of the entrance)