Here's an example of giving a Pokémon an item (probably not the recommended way though):
$Trainer.party[0].setItem(:REPEL)
This would make the
FIRST Pokémon hold a Repel.
If you want, you can add a message before or after, as this method does not display any text when making it hold an item. If the Pokémon is already holding an item, it will not take that item away first -- it will be deleted. If you could be more specific in that regard, that'd be nice.