firstly ask a career advisor if you have one if your school, cz they can give you all the info that u need,
im really intersted in psychology too, and to do psychology in university, which is what i was going to do, but now im doing a pyschology and criminology combined course,
but anyway, u need psychology at A level, and after looking at unis last year, i found out most uni's liked people to have science subjects like maths, biology, instead of subjects like geography. so biology, maths, and psych are useful. i did maths to AS level, and then psychology, biology and art at A level, and i got offered places at really good uni. maybe sociology may help as well.
to become a clinical psychologist, u need a Bsc Psych degree rather than a BA Psych degree, although the majority of uni's do the BSc rather than the BA. basically if u do a BA, u have to do an extra yr, if u want to become a clinical pyschologist.
for a criminal pyschologist, it may be useful to do either a combined criminology and pysch course, like im doing, or do a pysch degree, and then do more yrs at uni, doing a criminology degree as well. i have to admit, not a lot of uni's do the combined course tho - ones like sheffield hallam, coventry, huddersfield, but depending where u wana go etc - so look on uni websites, but u have another couple of yrs before uni yet. talk to teachers in sch - they can give u help on stuff like this!