How do I get started making unit tests for my Android app?

This seems like a silly question but I have an app in Android Studio and I'd like to try some TDD/Unit testing. I tried adding a tests folder and adding a class manually in the explorer but found I can only add Java classes under my main src folder. How can I simply add a tests folder at the same level as my src folder and create Java classes for it? Do I need something in my build.gradle?