Simple Label

<TextBlock FontSize="20" >My Text Block</TextBlock>

With a name so you can change it

<TextBlock Canvas.Left="20" Canvas.Top="100" Width="300" Height="200" 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"