As everyones saying, think about taking Psychology for A Levels. Its a wonderful subject and definitely my most interesting! Other subjects that compliment it are maths if you're going to be doing a LOT of statistics and in terms of worrying about whether they start from the basics in psychology, they do! You'll be in the same boat as everyone that hasn't done it before. I'm currently applying to do Psychology for my undergraduate and most universities want you to have done at least one from Pschology, Biology, Physics, Maths and Chemistry at A level. If you're thinking about clinical then it might be worth doing Chemistry but its not essential. I never thought about becoming a Psychologist but I've worked with a lot of them during my work experience. A vast majority have not only completed their undergraduate degree, but have taken their phD/ completed their doctorate and become doctors. To be able to start in a clinic and start having patients, you have to take on additional training. Not really sure how this works for countries other than the UK, but my supervisor got a funded to do her training course in Liverpool and she said the acceptance rate was only 2%! From your post you sound passionate and i have no doubt that you can achieve this. Good luck in the future x