Ask HN: Best approach for adding an API to an existing rails app

Hey, built this prototype game http://www.vvx.io/ - think it could be fun to allow API access so people could create bots that trade on the exchange. (e.g. scrape reddit for youtube videos, seed and buy/sell them onto the exchange.)

VVX.IO was my first full end-to-end just-me programming experiment. I was up until this a front-end and product guy (don't even code in my day job - the game is a part time labour of love) - so approaches to an API are new to me - there seem to be a lot of options and guidelines out there for offering APIs - from 3rd party services (mashape) to best-practice on auth etc. - what would you guys recommend as a good place to start?

Thanks a lot!