Hello, great question: the best thing you can have is a passion for computing and technology! Best A-levels to study are usually Mathematics, Computing and the sciences. Mathematics because it provides problem-solving and analytical skills, as well as providing the mathematical foundations for computing.
With regards to Computing and ICT qualifications at school, there is a wide variation in the topics studied on these; some have too much focus on using applications and technologies, rather than explaining how they work and deeper underpinning concepts. Some programming experience is helpful, but not essential.