You MUST visit a unit. I can't over emphasise that. Do not take no we're too busy as an answer. When you visit note runways, airlines, destinations, type of ATC operation (number of positions etc). Also be familiar with a radar display, what do the labels show, what a flight progress strip is and what info it normally contains. Be flexible in your answers, if you don't know an answer they will try and lead you towards it and look to see how receptive to instruction you are, but then they may also see if you are a "blind" follower, someone who just does what they're told irrespective of the dangers that could be inherent. Have an idea about general aircraft types and general performance, which will egt to 20,000 ft first a C152 or LJ35?? That sort of thing.
The computer tests are not too taxing...if they still do the "keep the numbers from colliding" one, don't worry if you can't do the maths questions at the bottom, they are just there to put you off...I'm pretty sure I didn't get one right (I also had several collisions!!) so don't worry if your not perfect....ATC is a very imperfect art ( I don't consider it a science)
the personnel interview is probably the worst bit and there's nothing you can do to prepare for it really. iTS STUFF LIKE, DO YOU CONSDER YOURSELF A MATURE PERSON?You obviously say YES, then the next question is always "Why?". just answer the questions, don't try and be funny, but be confident without being overly so, speak up etce etc...all common sense stuff.
Good luck...if you have aproblem with the visit let me know in this forum and we''ll sort something out