hi, I'm the same age as you and came onto Pprune with exactly the same questions as you. It didn't take me long to realise that it takes a lot to be accepted onto sponsorship schemes and the chance doesn't just jump into everyone young wannabe pilot's lap - you need very good GCSE and A level grades (if you're in UK) as well as loads more - but even with those things it's still VERY difficult to get onto these sponsorship schemes. I'm not trying to put you off at all, I would still love to be an airline pilot, the same as you do. Sponsorship does always seem to be immediately the best option, but often it isn't. Sometimes you'll go through the training, often around 2 years long I think, with no job at the end of it. Even if you do get a job with the airline straight away you still have to pay back the full cost of training, plus more - it isn't as 'free' as some people will want you to think. You will also be stuck with that airline whether you like it or not for up to 7 years, not a nice thing to have in your contract if you hate working there. Another thing is that there are very few companies doing sponsorship at the moment so there are thousands of applicants for maybe 100-200 places.
Sponsorships can be the best, easiest way to get into airline flying but there are a lot of drawbacks that people don't notice, or don't want to notice, when they first look into the idea.
What I have decided on though, is to get my licences (PPL, CPL and whatever else I might need) at my own pace while working at something other than flying, then get a job instructing, air taxis, sightseeing flights, anything, there are A LOT of jobs for pilots other than at airlines. Then once I have enough hours/experience/licences I'll apply for jobs at airlines *IF* I'm not enjoying my other flying job or if I still see airline flying as a better prospect than the one I have then.
Reading through this it does look like I'm a miserable goon who just wants to run down everyone's hopes, I'm not, its just that by reading other threads on this site that I would probably be better off taking another flying job when qualified while building up experience to take that airline job I eventually want. I believe 'direct entry' pilots are much better off in the long run than those taking the sponsorships.
Hope that helps.