As an Sociology student,and having a parent as a teacher of the subject, I can absolutely confirm Sociology and Pyschology are extremely different. Sociology studies the effects of the social environments of society on an individual, whereas Psychology looks at the biological aspects of human behaviour. They are very loosely related, and I'm almost positive you don't need to study either at GCSE to do it at A-Level, as the earlier comments said, many schools don't even offer the courses. Just make sure your Maths/Science grades are good, as it will help you when you get to study it at A-Level!