Now, I was around the same age when i wanted to start Game Development, let me give you this advice:
By all means follow thew advice above, but above all else. IT IS A SECONDARY GOAL. you NEED good results, and Game Design / Computer science etc. is not a huge part of Pre-16 curriculum.
There is a lot to learn in game development, algorithms to learn, programming techniques, design principals etc. If you at 14 spend your time learning all of this, you spend less time getting the results you need to get into University/Collage.
Don't' do what i did and spend your free time making games and neglecting your work, SCHOOL is your first priority, friends/Life come before games at your age. you'll ahve plenty of time to learn how to-do all of this properly once your in a Uni. so don't let it get in your way before hand.