Any Help On Guis?

I was wondering how i can make a GUI on the left down corner and make it compatible with all screens because i made one and i tried putting it down on the left down corner and when i played it went up instead of where i put it in studio. Also when i press F11 to go in Fullscreen mode it moves down or up and when i get out of Fullscreen it moves up and down where it was before - if that makes sense ..

If you can help me, that'd be great thanks!

Under the GUI's position try to use coordinates in this manner. "{0.4, -100},{0.9, -100}" The RIght number should be negative. And the left number should be a decimal.