You need Wi-Fi connection to do that.
Exchange friend codes with a friend and start sending messages to each other via the game's Moogle mail system (via Internet, not just wireless). You can send one message per hour but that can easily be tricked by changing time settings in your DS.
When you have at least 7-8 messages sent and received (I think the exact number is 6, but you want to be sure), start sending letters to Topapa. After a while, he'll tell you all kids in the village have disappeared and ask you to go investigate.
When you've talked to him, he'll send you to the altar cave (the very first dungeon in the game), and there you will fight an (easy) boss and save the kids.
Voila, Onion Knight is yours!
It's only worth it if you are level 90+ (character level, not job level), so refer to my threads in this section for Capitain Maxx Power's awesome tips on how to level up quick.