Build for Google Cardboard
Read this section if you are targeting Google Cardboard on Android and/or iOS.
A good starting point is the Google Cardboard for Unity, Get Started guide (for more information, visit https://developers.google.com/cardboard/unity/get-started).
The Android setup
If you've never built for Android, you'll first need to download and install the Android SDK. Take a look at Unity manual for Android SDK Setup (http://docs.unity3d.com/Manual/android-sdksetup.html). You'll need to install the Android Developer Studio (or at least, the smaller SDK Tools) and other related tools, such as Java (JVM) and the USB drivers.
It might be a good idea to first build, install, and run another Unity project without the Cardboard SDK to ensure that you have all the pieces in place. (A scene with just a cube would be fine.) Make sure that you know how to install and run it on your Android phone.
The iOS setup
A good starting point is Unity manual, Getting Started with iOS Development...