The most important qualification will be the pilot's licence. This takes stacks of money.
You will need to be fluent in English and, depending on which airline you go for, may need 5 GCSEs A*-C + 3 A-levels (BBC or higher). Or, produce a 2:2 degree or higher. Other airlines have lower requirements, so this would be the "worst case" scenario.
You will need to go and get a medical exam taken. The CAA website will have all the information relating to that. You will need a Class 1 medical to fly commercial passengers.
You could start learning to fly now, but it is expensive and isn't suitable for many people until they are in a full time job. You should look at joining the Air Cadets as this will give you flying experience and other useful skills. It would also be very important to get a solid education while you can (such as solid A-levels, or even a degree). If you lose your job, or fail a medical exam and can never fly again (it might happen in 5 years or 25!) then you need to have a backup plan.
Check out the British Airways future pilot scheme. It is closed at the moment, but should run for a few more years. It will give you a lot of useful information (such as the flying schools they support) and you can read up on your own.