All depends on what type of counselling that you want to go into.
I think the basic qualification that you can do is a Diploma in Counselling and Psychotherapy, and that'll make you basically qualified... but the more qualifications and letters you have the better really, if you want people to come to your door.
for alevels:
Psychology and Philosophy are two subjects that would be good for you.
Psychology for obvious reasons, how the brain/mind works etc etc.... Philosophy is good because it'll make you think about ethics and enable you to help a client to look beyond the question or problem that they may be having difficulties with. Existentialism is really interesting.
Sociology might be a good choice too, as that'll give you an insight into all the cultural implications that might affect how a person thinks/acts and behaves. such as religion/media and the importance of family structures and a smidgen of anthropology as well.
Drama would be good for the confidence of speaking to people, and if you wanted to be a cognitive behavioural therapist, it would give you a good range of skills for various therapies....
at degree level theres Psychology and Counselling degrees which would be a good place to start. Theres a person on here under the username ~sweetness~ who is studying that. she might be able to provide you with more answers....
Do you know what type of counselling you'd like to do?