The standard icons pack for Windows Phone 8 you can find here:
Dark Theme
C:\Program Files (x86)\Microsoft SDKs\Windows Phone\v8.0\Icons\Dark
Light Theme
C:\Program Files (x86)\Microsoft SDKs\Windows Phone\v8.0\Icons\Light
Add an image to the project.
In Xaml:
xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone"
<phone:PhoneApplicationPage.ApplicationBar>
<shell:ApplicationBar>
<shell:ApplicationBarIconButton IconUri="/Assets/add.png"
Text="Add"
Click="AddAppBarBtn_Click" />
</shell:ApplicationBar>
</phone:PhoneApplicationPage.ApplicationBar>
In cs:
private void AddAppBarBtn_Click(object sender, EventArgs e)
{
// Code
}