If you want to add a tooltip to a control you must first check if it allows the ToolTipService namespace. If yes, you can add the tooltip the following way:
Xaml
<Button x:Name="MyButton" Content="Hello!" Width="50" Height="30">
<ToolTipService.ToolTip>
<TextBlock x:Name="ToolTip" Text="Click me!"></TextBlock>
</ToolTipService.ToolTip>
</Button>
C#
ToolTipService.SetToolTip( MyButton, new TextBlock() { Text = "Click me!" } );
You can also use another control instead of TextBlock, for example Image control or Rectangle control.
That's it!