Add simple form into a page

Hi,
I want to add a simple calculation form into one of my page.
Example: User enters 2 number, click Calculate and the result is displayed on the page.
ex: a + b = c

To prevent a reload, I tough I would simply use Javascript.

But I am sorry to admit that I am not able at all, I cannot make it work.
Can someone tell me a simple way how to add do it? I am ready to use something else than Javascript. I just want to get that form to work !! I have spent to many time on it already.

Thanks in advance for your help !!
Nancy