Pokémon HeartGold and SoulSilver/Gameplay

Catching Pokémon
After you've delivered the egg to Professor Elm and left ../New Bark Town/, you'll be given a tutorial on how to catch wild Pokémon by your friend. They'll show you that you should weaken the Pokémon until its HP is in the red or yellow, and then throw a Poké Ball. If you can, try giving the enemy a status, such as sleep, freeze, or paralysis. These will increase the odds of capturing it. Pokémon that have been caught at least three times and Pokémon acquired through trades will be easier to catch, as well. You cannot capture another trainer's Pokémon, and you cannot catch a Pokémon while in a double battle.

The type of Poké Ball you use will also affect whether or not you're successful at catching a Pokémon.