Either a PP Up or a PP Max can increase your PP. PP can be restored using items, such as an Ether, which restores PP by 10 points. If you can’t find an Ether, it’s worthwhile to see if there’s any Leppa Berries around, as they will restore your PP by 10 points for one move. If your PP is completely deprived and none of the aformentioned items are available, it is wise to visit a Pokémon Center instead.