Using ToolTip for TextBox with TextBox.ToolTip and ToolTip tag

   
     

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      HorizontalAlignment="Center" VerticalAlignment="Center">

<TextBox Width="100">
  <TextBox.ToolTip>
    <ToolTip Content="Type something here" />
  </TextBox.ToolTip>
</TextBox>

</Page>