CBT is compulsory basic training, no tests involved until you start moving up in licence categories. Personally i would go for a geared bike rather than a twist and t##t! This allows you to ride either, if you do your cbt on a moped thats all you can ride. Considering your age, get ypur cbt done and get a bike, anything japanese is good, anything else isn't worth mentioning. Yamaha, suzuki, honda, best bet is to go to a bike dealers and try sitting on a few and see what you find comfortable, for me it was a suzuki gz125, cruiser styled, low seat height, good for me as i only have little legs! As far as post cbt, get a couple of years experience riding, then you have two options, either do your cbt again and wait till you turn 24 and do the full cat A tests, theory, mod 1 and mod 2. Or, when your cbt is almost up say 6 months left you can do an A2 licence which allows you to ride UP TO 46.6BHP, problem with the A2 licence is if you want to then upgrade to a full CAT A you have to do another set of tests!
Oh and anyone who says their 50cc twist and t##t does 69mph is talking horse s##t BY LAW they are restricted to 28mph!