1) Add a timer to your form, either via the designer or via code.
2) Set the Interval property to 100 - this will cause 1/10th of a second updates.
3) Handle the Tick Event.
4) In the Tick handler, set the TextBox.Text property to DateTime.Now.ToString(formatString), where teh formatString defines what info you an't to display. Have a look here: Formatting a DateTime for display - format string description
] for the format possibilities.
Doing the actual code is trivial, so I will leave it as an exercise for the reader!