Inherited dialogs and Value from other Form

Hey, 

I've got to make a program to calculating a fields in which main form will have 3 buttons "Square" , "Circle" and "Rectangle" after pressing one of them program should appear a new window which ask for values , like for Square , how long is side of square , and button which is confirm after clicking the button this new window should close or hide and  in main form should appear a calculated field. To the soulution I have to use Inherite dialogs. My problem is that I can make Main form , but I can't get how to use value from second window without using textbox in first one and how to use this inherite dialogs I'm just adding windows and making buttons from main form to show them after click. Thanks for help sorry if something is not understandable but my programmer english isn't that good :(