If you already have Animal Crossing: New Leaf as a cartridge or downloaded to your system in the Nintendo 3DS family, you will be able to use the amiibo features by downloading the free Welcome Amiibo update in the Nintendo eShop.
However, if you do not have Animal Crossing: New Leaf yet, you can buy Nintendo Selects: Animal Crossing: New Leaf Welcome amiibo from the Nintendo eShop on a system from the Nintendo 3DS family or buy the physical cartridge from a retailer.
If you already have the game and are asking about how to use amiibos with your game, make sure you have the NFC reader (unless you have a New 3DS). There are various amiibo features within the game–click here for Nintendo's official page on the amiibo features in New Leaf!
Once you do, look around your town for a genie lamp. It'll ask you to take it somewhere (your house) and put it inside somewhere (doesn't matter where). Touch it and Wisp will show up and ask you to "give him a body". At that point, place your Amiibo on the lower screen (make sure your internet connection is active).
I set the laser printer on Stun
Fall down, go boom