Android Studio Unit test guide?

I’m trying to set up Android Studio projects to work with unit test. Despite several option inside (JUnit, Android Test) and outside (Robolectric) I can’t manage to have a working solution.

I know I’m not good in gradle-maven-junit-java architecture, and I’m looking for a basic guide that explains the meaning of all this together. You can’t control without understanding.

Do you have some tips or guide or claryfying tutorial to make me go on? I’m even not sure that this question is good for stackoverflow, otherwise I apologize.

Thanks