* Go to your SDK Manager and make sure the latest versions of "Google Play services" and "Google Repository" are installed. When you do that - make sure to not install any other updates. The SDK Manager likes to suggest all kinds of updates that you don't need and will use up all your disk space.

* See [https://developers.google.com/android/guides/setup here]. Note it's talking about the build.gradle file inside the app subdirectory of your project.