If you visit a unit or two and chat to the ATCOs on duty, theyll steer you in the right direction. You need to proove a general interest and understanding of what goes on at a Unit, plus show you have the right qualities needed to make an ATCO. Apart from that any knowledge is good, just dont bulls**t or theyll let you dig an even bigger hole to jump into. Be calm and concise in your answers and dont dither. ATCOs should be alert, able to handle any pressure situation, decisive and responsible. (although maybe I slipped through the net there!!!)
Good Luck