Handle the KeyDown and KeyPress events of a textbox.
Check if every key pressed is numeric.

Teme64 points you to a thread where a question is asked some code is given and what was already done.
What you are saying Eyo is : "I HAVE AN ERROR, SOLVE IT!" That's why I came up with the car...

My car won't start!!! Help plz.

Which car?
What are the circumstances in which it won't start?
etc.
etc.

Please refrase your question.