In this article you will see how to set the Start page in a Windows Phone 7 application,
Very often I see people struggling to set the start page for a Windows Phone 7 application. By default MainPage.xaml serves as the start page. If you have many XAML files in a phone application you are creating, like below:Now you have three XAML files in the Windows Phone application project. If you want to set Page2.xaml as the start page then open the file WMAppManifest.xml from theProperties folder in the Solution Explorer. In XML file change the default task Navigation attribute to Page2.xaml:
Windows Phone Development Step by Step Tutorial