The field test, which is meant to improve the game features and balance, will support Android version 4.3 or higher, and iOS devices iPhone 5 or higher. A Gmail or Google account is also required to register.
Sign-ups are available here. If you’re not in Japan and want to sign-up for the field test, you’ll need to do so via a proxy. Though, given the game has to do with catching Pokemon in the real world, we can’t say whether or not it will be active in your location.
Pokemon GO was first announced in September 2015.