Android Testing Initiate

I want to learn android application testing(Unit testing as well as other testing) . I just created an app in android studio and found that there is already a package for it and also a class within it(Named ApplicationTest). Now I searched a lot which tells me how to use this ApplicationTest class (for Unit testing as well as other testings), But, didn't found any correct answer.Could you please tell me how to use this,or any link which guide me to achieve this. Thanks in advance.