Iv'e read on here about using a cut up credit card to get the sandard stereo out (search the threads, theres plenty of info on here) but removal pins are cheap enough to get from somewhere like halfords. Slide the remova pins in and the stereo shuld come straight out. Thejn just disconnect it etc...
Regarding the gap thats left, you will need a fascia panel (again from somewhere like Halfords) which fills the gap with a cubby hole. It depends on what your going to stick back in there i.e if its a motorized front or a double din dvd player I think.