First up you'll need your CBT, there will be a 'rider training' type place near you, just need to find them and get the costs, they'll let you hire a bike and all equipment, at a cost of around £90-120 ish
Once you've got your CBT then you are able to ride a restricted 'learner legal' 125cc motorbike, on L Plates. You won't be allowed on motorways or to carry a pillion passenger
You can just 'skip' that stage though, book your Theory and then do your Direct Access test, not sure on how much the theory costs but not a huge amount, the Direct Access itself will be £500+ including equipment hire, and that several days...
Once you've got your Direct Access you can ride any motorbike (seeing as you're 21 you skip the restricted phase), however obviously a 1000cc sports bike probably isn't the most sensible first bike
Some rider training places will do the CBT and Direct Access in one combined week long intensive course.
Last thing to note really is equipment, you're talking a good couple of hundred for 'cheap' stuff, the more expensive stuff gets very expensive (£400 for a helmet for example), but it's worth getting decent stuff
Hope that helps, i'm 21 (almost 22) and did my CBT last september, recently bought a 125 to commute to and from work with (63mpg \o/) and hope to do my direct access later in the year and either derestrict my current bike for a tiny bit extra power or buy a VFR400 depends how much money i have