Law doesn't have prerequisites because the entry mark is so high and it doesn't require maths.
After the HSC I was given a scholarship interview with a law firm. They asked me "who's your favourite judge?" and I had no idea. I didn't get the scholarship.
Moral of the story: if you want to really do well in law, immerse yourself in it now. Read books by famous lawyers (eg. Geoffrey Robertson, who writes for non-lawyers), judges (like Michael Kirby), trials (eg. the Anita Cobby trial), cases (eg. Mabo), Australian legal history, legal philosophy, whatever about law interests you. There are lots of free law journal articles online, just type a subject and there'll be an article on it. I also suggest buying some cheap second-hand law textbooks from second-hand bookshops and learning them already. In university you're thrown in the deep-end with lots of textboks and lots of pages to read and with noone to hold your hand to ask if you understand it or even help you understand it. That's what makes law, and university in general, so much more difficult than high school.
Austlii.edu.au has real legal cases. They're very difficult to understand but give it a go because even if you don't understand most of it you'll start asking yourself questions of what this and that means, and if you're lucky you'll find someone to give you answers. Austlii also has lots of law journal articles.