Pokémon Ruby and Sapphire/Pokémon Contests

Pokémon Contests involve going to contest halls and going through two rounds of judging based on condition and moves. To enter your first contest you have to obtain the Contest Pass from one of the people in the Normal Rank Contest Hall in Verdanturf Town, although the first contest hall you see is the Hyper Rank in Slateport City - you obtain the PokéBlock Case.

Preparation
You might want to pick a Pokémon that has moves that are the same type as the contest they are participating in.