Simple Label
<TextBlock FontSize="20" >My Text Block</TextBlock>
With a name so you can change it
<TextBlock Grid.Row="2" Grid.Column="5" FontSize="20" x:Name="MyTextBlock">My Text Block</TextBlock>
MyTextBlock.Text = "abcd";
Newline
Use the hexadecimal encoded value 
 which represents the line feed.
Text="Line 1 
 Line 2"Feel free to comment if you can add help to this page or point out issues and solutions you have found. I do not provide support on this site, if you need help with a problem head over to stack overflow.