Help in developing Android applications.

Hello everyone, I hope you are fine.

I want to make an inquiry about android application development, because I need to create an application that can capture the battery consumption in smartphone, and read the basics and programming in Java but still not know where to start, if anyone have idea of codes that serve me, help me sources, or any type of information related to this topic, I appreciate it.

I should design the application should look like Battery Mix Battery or Monitor Widget.