Firstly you will need to do your 1 day's 'cbt' course (compulsory basic training) and can expect to pay ~£100. A local riding school should be able to provide you with the appropriate protective clothing and a bike to do you're training on.
Assuming you're at least 21, probably the best option would be to do the 'direct access course' if not, there's quite a few other options which can all be found on here;
http://www.direct.gov.uk/en/Motoring...vers/index.htm
Then you will need your;
Theory test: £31, purchase the highway code and swat up, particularly if you don't already drive a car.
Module 1 Test: £15.50 + hire of school bike bike (Skills set through cones and manoeuvres basically)
Adequate training - how much training you need depends on how quickly you pick things up, and what your riding instructor recommends... much the same as learning to drive. Personally, I had four full days training and then test on Friday. To be honest I can't remember exactly how much I spend on training, somewhere in the region of £500...
Module 2 Test: £75 + hire of school bike
Once you've passed your test and you know motorcycling is for you... time to buy the gear and of course your bike
Ideally, you want Helmet, Gloves, Boots, Trousers and a Jacket... spend as much as you can afford... and in my opinion avoid second hand stuff, particularly helmets.
As for bikes, unless you're made of money, I wouldn't buy new that's for sure. The choice of bike is up to you... but if you're not 17 and plan to use it for stuff other than a commute across the city, you will be wanting to avoid 125's and move to bigger machines as you will get bored very very quickly, the ninja 250 from what I've heard is a waste of time, suzuki bandits, honda cbf's and other such steeds should see you right... myself, I went from a Honda xr125 to a Honda cbr600 and so far so good - love it.
Listed above will all be good strong first bikes and seems to be what a lot of people go for. Just go steady as you will still be learning once passed and it's easy to say your responsible but all that power is just a twist of the wrist away!
Good luck
