Im making a gui for my opengl app and I am just wondering how to change the button appearance when the cursor is above the button, like starcraft. Thanks

Well, you would have to handle the WM_MOUSEMOVE message and then check wether the pointer is in the area of one of your GUIs buttons. If you know over which button it is, you can simply switch for example the texture for the button, use another model or playback a small animation...