If it is an existing program, then basically you can't. The developer chose to not provide the resizing functionality.
If it is an application that you are programming, then choose an area in the form that you will use as a "handle", and then intercept the mouse down/up and mousemove events to manually change the width and height of the form. It is not possible to provide
a more detailed answer because you didn't indicate the technology that you are using for your application.
Since you posted your question in the Training and Certification
forum, I'll provide you a couple of links into the training page for Visual Studio to find courses that teach you how to program desktop applications:
http://www.microsoft.com/learning/en-us/course.aspx?ID=40362A
http://www.microsoft.com/learning/en-us/course.aspx?ID=10262A
