Get a good start in secondary school. Take at least one foreign language and pursue it. The better your language skills are the greater your chances of being chosen for international flights. Indicate your desire when it is time to do your schoolwork. You may be able to do it though a travel agency or even an airport.
Start researching the airlines and their requirements. Many of the major carriers such as British Airways require a college degree. However, some of the smaller ones will take GCSE's as long as you performed well. Be willing to work in a smaller carrier such as British Midlands, Easy Jet or Ryan Air. They will give you valuable experience and a greater chance of breaking into the big carriers.
Finish year 11 and your GCSE's then look into colleges that offer a travel and tourism program. This will give you a leg up against most of your competition. This is a fairly popular diploma and should be readily available.
Look at airlines and their openings. Some will take people straight out of school and put them through their own training program. These can take from four weeks to several months so be prepared to put some time in. Airlines generally want people who are at least 18, but it never hurts to check.
Be flexible. Once you finish your chosen route you will likely be put on a reserve list. This means you may get a call at a moment's notice and be expected to fly.