Hi Magpie, and welcome to the window cleaning boards.
If you're in Scotland, you'll need to be licenced, but I don't think the rest of GB has caught up with us yet
If you ARE in Scotland, contact the Licencing Section of your local council for application forms. You'll need to supply the fee, a copy of your unbought Public Liability insurance (say you'll buy that once you're licenced!) and depending on where you are, a couple of passport sized photos. Some whizz-bang licencing sections have their own digital cameras so you'll need to go to the offices in person.
If you have an unblemished background, you'll hear nothing until you get the licence through the door. If you have a police record, it depends - charges of theft and violence are strictly rejected - you might get a visit from a friendly sergeant to vet you.
If you're of the English/Welsh persuasion, contact the council anyway just to see what the local picture is.
A good thread to check (if you haven't already done so) is
http://www.cleanitup.co.uk/cgi-bin/yabb/YaBB.pl?board=Startingwindowcleaning;action=display;num=1064338611 a lot of reading, but worth it.
All the best - have fun - and let us know how you get on
Davie