Scan through the previous threads in this forum about this, you'll find there are loads of them, and get a good picture of what to expect. In some of them you'll find a link by Goldfish Watcher to a site he's downloaded loads of info on. (if you're lucky, he may even post it on here at some point).
Secondly get along to at least ONE place and witness ATC in action. If able, do both a NATS airfield and centre, and also a non-NATS place and compare and contrast. For the NATS places, the recruitment office in London can give you contact details.
Thirdly, if you don't know huge amounts about ATC (ie not a spotter or descended from pilot/controller etc), go to the library and peruse any ATC and airport books they have, background knowledge is important.
easily found the link by Goldfish Watcher, goto:
thispage
and have a look!!