You can find some of the best Android projects and can work on them these projects you can follow on to the GitHub:-
Amazing Open Source Android Apps.
If you want to know more about GitHub whether if you are an absolute beginner or you have a little knowledge of GitHub in that case to learn GitHub and all its concepts such as pull request or creating a repository in detail which is necessary for a programmer you can learn it by taking up the GitHub training course. If you like to read then you can have a look at the GitHub tutorial. If you want to learn it by watching the video tutorial then you can have a look at the following tutorial:- .