One of the more unique features in The Legend of Zelda: A Link Between Worlds is the Streetpass ability to send your created Shadow Link to other player's worlds, and battle them for Rupees. Luckily, if you fall in battle, you suffer absolutely no penalty, however, depending on how outfitted the Shadow Link is, if you win, you get a nice bonus of Rupees.
Shadow Link Streetpass isn't available right from the start of the game though. You'll need to get past the tutorial dungeon and wait until you're able to explore the world to unlock it.
Getting it is really rather easy, but if you're not the type to talk to every NPC you come across, it could be easily overlooked.
To unlock Shadow Link:
First make your way West and head into Kakariko village. If you've already been there and you've unlocked the Weather Vane, you can choose to fly there by ringing the Bell.
One in Kakariko Village, head West toward the empty plot of land underneath the Shield Shop. An old man will be waiting for you to unlock the Street Pass functionality and allow you to customize your very own Street Pass Link.
Whenever you Streetpass another player with a Shadow Link, you will find a sign on that plot of land which will state where to find him.
Customizing Shadow Link:
You can only customize Link with items you're currently holding. That means if you haven't rented an item from Ravio, you won't be able to equip it. The better items you give Shadow Link, the higher his bounty will be for defeating him.
Link doesn't have to just hold weapon items, he can hold restorative items as well, such as potions or Premium Milk.
Make sure to play around with combinations to see which have the higher bounty, since that directly affects your Shadow Link's difficulty in battle.