Upon entering, head to the right. The little boy will tell you about hidden items. Check the upper left tile of the blank space in the patch of grass to obtain a Potion. Exit south and keep going down past the lady to pick up a Poké Ball. Jump down to the left and enter the forest again.
Head left this time to battle the Bug Catcher. Defeat him amdpick up the Paralyze Heal behind him.
Bug Catcher Lyle(Prize Money: P80)
Pokémon:
Continue by heading to the right until you see a researcher. Approach him for a cutscene and a battle.
Team Magma Grunt(Prize Money: P360)
Pokémon:
Defeat the grunt and the Devon Researcher will give you an Exp. Share and he will also fully restore your Pokémon.
Head to the left and battle the Bug Catcher at the end then make your way up and pick up the Ether.
Bug Catcher James(Prize Money: P128)
Pokémon:
Head for the exit for now as you cannot proceed to the right due to the thorny tree. Just take note of its location and remember to come back for it later when you get the HM Cut.