Basic science and maths are a good start, in addition to that, it is important to be very curious about the Earth and to be interested in geology and rocks.
For university study it depends on where you want to study. If you want to come to Oxford, then we need A-level maths and physics, then either chemistry, biology, geography or geology. Other universities have different entry requirements. I did my geology degree at Birkbeck College which is part of the university of London, just ask for for 95 – 120 UCAS points or equivalent and have no specific subject requirements like Oxford