The input command will create the input box for you and will then wait for the user to type something in the box and press enter. The program will then take what was typed and continue. You could change the value of a variable using what was typed in the input box. The information returned is a string so you would need to convert it to another type if you wanted it to be an integer or something else. You would also need to add some error checking to make sure what was typed in was a number, if that was required.
I suggest you start doing some of the very basic tutorials, they show you input boxes and many other simple but useful things.
EDIT:- I see luskos has already given you a working example
http://forum.thegamecreators.com/?m=forum_view&t=127250&b=7. Did you even try the example?
