How to minimize whole application in android?

I'm not sure what you mean by minimize. If you want to hide your app and present the user with the homescreen you can use the following intent.

Intent startMain = new Intent(Intent.ACTION_MAIN);
startMain.addCategory(Intent.CATEGORY_HOME);
startMain.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
startActivity(startMain);

Although the Home button is more than sufficient if a user wants to hide your app