How do I start I , download the android studio. But I don't know how to write code what is the source and how can I find specific ? Like what is the code for a white screen ?
Go learn Android development on Udacity or Youtube and also learn yourself Java or Swift depending on what device you want your app on. You can't just copy a source and expect it to work. You need to know the fundamentals of object oriented programming and know the syntax of the language. Once you know a programming language you can teach yourself another without getting too perplexed. Just even search Youtube videos or something. But if you're like me then you rather Read / Write. I suggest if you don't already know find out your learning style.