Well, you just "draw" your gui, no scripting yet, it's the easy part.
If you want to make a button do something, You make the button, Click it, On your right there will be some boxes, One of them is command, Enter DoStuff(); here
Save your gui, and in your script you do this:
function doStuff()
{
MessageBoxOK("Cookies4U","You click'd da button :D");
}