You can do it very easily with jQuery. Check this quick example I just made:
A Pen by Captain Anonymous
As you can see, after 200 pixels scrolled, the div "modal" is called. This is basic HTML/JS, very easy to customize. You can combine with Cookies to not show the modal more than once if it has been closed or even use a modal plugin such as Colorbox.
I highly recommend you to use Stack Overflow, there are plenty of web developers ready to help.