Is it possible to autostart my app at device boot having windows 8.1 or windows phone 8.1?
If yes, how?
It is possible with Android using BroadcastReceiver
It isn't possible to automatically launch an application at foreground on Windows Phone and Windows 8.1 (be it at startup or at any other time). However, your application can execute in background (given some limitations) by using background agents, and interact in some ways with the user (for instance, by using toast notifications).