A car licence counts as a provisional licence, except that you can ride a 50cc scooter restricted to 32mph without a cbt. To ride up to a 125cc bike you need to do your CBT, which is not a test, this will permit you to ride any bike up to 125cc which has less than 14bhp (I think, somewhere around there).
For either of these you MUST ride with L plates, and not use motorways or carry pillion passengers. It doesnt matter that you have a car licence, you must still go back to those L plates. Then of course, you need tax, mot and insurance for your bike like with any other vehicle.
To progress further, depending on your age you can go 1 of 2 ways.
Firstly, if you are under 21, you can book and sit your theory test (again, having a car licence makes no odds as the theory questions are different), and then book and sit a practical test, in the same way that you get a car licence. Provided you pass both of these, you will be given a restricted motorcycle licence as you are under 21, this entitles you to ride any sized engine provided it is restricted to 33bhp or less, and you may use the motorways and take passengers, you also get to ditch the L plates at this point.
The restricted licence lasts for 2 years, and is then upgraded to the unrestricted licence without sitting another test, allowing you to ride what ever you want. So it is possible to ride any sized bike aged 19+.
There is no way to upgrade it sooner than 2 years, unless you turn 21 during those 2 years, in which case you can sit a further practical test to show your competence and have your licence upgraded to a full licence.
However if you are already 21 or older, you can sit a course which incorporates all the tests and training, usually lasting 3 - 5 full days. This course is the direct access course, which includes doing your cbt, your theory and practical tests with the required training inbetween. This will give you your fully unrestricted licence straight away and costs (at last check) £400 - £500 which includes all associated fees.
However, a word of warning - if you do sit the direct access course, please think very carefully about which bike you want to ride. They are completely and utterly different to handle on the roads in the dry, let alone in the wet, and there is no way a 5 day course can teach you to control the power in say...a R1 or CBR1000, this has to come from experiance. If necessary, just ask to have the bike restricted, the restrictor can be removed at a later date when you feel confident enough to handle the power. You always have to remember that whilst bikes have significantly less bhp than cars, they are alot lighter and the power to weight ratio is alot higher. An R1 may have 120 odd bhp, which is about the same as an average family car, however it will literally piss all over a ferrari. You must respect this power, something that alot of people dont when they take the route of direct access, especially as you only have 2 wheels and surface contact which is less than the surface contact of your shoes. Not being able to control this power will be a danger to yourself and other road users, and will detract from your enjoyment.
Im sorry to lecture, but I, unfortunately have seen, heard and known so many people who have gotten a bike that is too powerful for them to control and they have ended up in a ditch. Not all of them got back out of the ditch afterwards. I would hate for the same to happen to you, riding a motorcycle is extremely enjoyable, but it is fraught with danger for inexperianced and even experianced riders, but more so for the inexperianced.
They all tried being the big men, as to who could handle the most power, and not all of them could control it sufficiently.
If you want to draw comparison to power of a restricted bike, when mine was restricted to 33bhp it would do 0-60mph in less than 4 seconds and reach 120mph.