There's no easy, quick way to become a Pilot.
You'll have to start working hard now and for the duration of your flying career. 40+ years.
At 15 I would suggest you do Maths & Physics at school then look at some sort of back up plan. I.e. do an apprenticeship in a trade like an electrician. Just in case something happens that means you can no longer fly. Loss of medical for example.
If you went on an integrated full time course at 18 then the chances are you might get into a jet or turbo prop when you qualify but you probably won't see a 747 until you're at least 30+. And that's if you're in the RHS by the time you're 20. Your first few years as a FO will be doing short haul stuff all around the country, getting up at stupid O'clock in the morning and working till you're shattered. You won't see the glitzy high life of long haul travel for many many years. Many people will argue that it isn't even glamourous anyway.
You could look at the Military too.
Keep you sights aimed low. Don't fall into the egotistical trap of 'I want to be an airline pilot and anything smaller than a scarebus or Boeing is no good'
If you get the chance to fly something and be paid to do it, don't turn your nose up at it.
Remain realistic, work hard, save money where you can when you're employeed and never give up. You patience will tested to it's extremes!!!
Good luck.
Chilli.