How do you get Pokemon GO?

You can download Pokemon GO for free at the Google Play Store using an Android, but your device's version has to be 4.4 or higher. For iOS, you can download it for free in the App Store as long as your iOS version is 8 or higher. Any device lower than the version required cannot download the game. (It also depends on your location, for Pokemon GO is only available in a few countries as of now.)
Go to your app store - it's free.