Exchange your Friend Codes, find out what's in your own FS, get them from everyone else.

REMEMBER: Being online at the same time as the friend you add is the only way to unlock Hidden Abilities and the third Pokémon slot (if your friend's beaten the game).

Also, if you want to help others but aren't interested in keeping a FS yourself, delete the person's card after unlocking everything and they keep the FS while you've made room.