How do I change my screen size?

I used to have a widescreen monitor hooked up, but now I have a.. squarish shaped one.. how do I get the screen back to that size instead of having it all stretched?How do I change my screen size?
http://www.computerhope.com/shortcut/ie.How do I change my screen size?
right click desktop %26gt; properties %26gt; settings %26gt; choose your resolution.
press the F11 button

mouse cursor is lagging