You need to send off your driving licence to get a provisional motorcycle entitlement added. There may be a charge for this, I'm not sure. Once you get it back you need to do a Compulsory Basic Training (CBT) course before you're allowed to ride around unsupervised. The course lasts a few hours and it's to make sure that you can ride the bike well enough and safely enough to be allowed out on the roads. Any motorcycle training centre will offer CBT courses.
I think you'll be lucky to pick up the bike for £1000, but fingers crossed eh?
. You'll also need to budget for kit; you need a helmet obviously, and gloves are essential too (never ride without gloves, your hands are too valuable). Sturdy boots are a good idea but it's not really necessary to buy proper motorcycle ones of you can't afford them. A decent leather jacket isn't essential but is a good idea- and a set of waterproof coveralls if you're commuting
.
Finally, get a decent lock and
always use it, a CBR125 can be lifted into the back of a van in seconds so if possible lock the bike to something solid by the frame (not the wheels which can be removed).