"Catching a Pokémon is quite simple," stated Shiranui, "What you need to do is find the wild Pokémon you want to capture, weaken it in battle and throw a Poké Ball at it.  There is a chance the captured Pokémon would break free, but you can improve your odds of a successful capture by using a specialised Poké Ball or inflicting a status condition on your target.

"Your Mankey may have a type advantage at the Rustboro Gym since Rock-type Pokémon are weak against Fighting-type attacks, but you will need to train both it and any Pokémon you catch as well; as a gym leader's Pokémon can be tough to beat.  I suggest you teach it Low Kick as it is powerful against heavy foes.  Any questions?"