This achievement is pretty self-explanatory, but it unlocks once you've returned to a hub world after collecting a total of 12,000 Coins. (Meaning that, if you're keeping count during a level, it won't unlock until after you've completed that level.) You will likely unlock this naturally during the story as long you are going for as many Coins as you can throughout the game. I personally refrained from buying any costumes until after I unlocked this achievement, I haven't tested to see whether the game keeps a running count or just goes by your in-game total.
Tip: I also noticed that the game seems to only scan for this achievement occasionally, so if you hit the 12k mark, and it doesn't immediately unlock, don't panic. Completing another level caused it to unlock for me after I experienced this. Entering and exiting a level may have the same effect.