SO you want to connect a button to the computer for that you also need to have a app on your computer where you can select the com port to which your arduino is connected to and then utilise a Serial Port component to read the bits from arduino.
The way of doing this in steps is:
Attach a button to Arduino(any one you want like a very simple push button or any complicated button you think you like) after that write an application in the language you know about like c# to accumulate all that information coming from Arduino that what to do when button on arduino is pressed.