the moreon twins looped in their first week (apparently.....)
everyone knows windsurfing is all about practice, and lots of it. so talent combined with a lot of practice is the key. there is a reason most of the UK pros have spent a lot of time abroad... its pretty difficult to get enough time on the water in the uk compared with 8 hours and day in some places. although at the moment in the UK this might not be true??
in terms of sponsorship its a bit of a misconception that its all down to rider skill, although this plays a big part what most sponsors a looking for are ambassadors. people who are fully involved in the sport, get a lot of coverage, go to events, write articles make videos etc etc. potentially you someone could be a sponsored beginner board rider promoting starboard gos and be crap at windsurfing....
likewise there are loads of kids who sail to a high level in remote desitinations around the world who struggle to find sponsors becuase its hard for them to promote products
If you're young and keen, the best place to start is probably your local shop. As Al said, you need to spend all your money on training and getting time on the water, maybe think about entering events and/or going sailing with better sailors to help push youself.
Sponsorship. I wouldn't worry too much. Many sponsored riders are paying Â£800+ for a board, when something on ebay could do the job for a third of that.. If you can blag/earn money from somewhere, that's much better. Good luck!