There are a lot of good you tube tutorials including that of "oracle developers , knowthen" specifically for learning real time app development.You seem not sure of which specific real time app you want to develop, you can consider and get an idea from some great real time app examples like
Cloud 9 IDE
Moreover, you can take help from realtimeapi.io during development.It specifically is about real time resources and it can provide you with great help.
developers.google is also recommended.First sort out what type of app you want to develop and then start the journey.
I hope this helps ,best of luck
Android App Development