Whilst it is till very early to be thinking about university, it is good that you are taking an interest at this stage. At this point if you can get work experience, that's great, but GCSEs are the most important thing you can influence right now. 7A* would be fantastic, although aim for the highest grades you can achieve rather than a certain aim, if 7A* is fulfilling your potential, that is fine. Bear in mind that while AS levels and A2 predictions will be important, GCSEs will be taken into account. I am not applying for law, but economics, where GCSE grades are very important, and a string of A*s will be a real help. With law being similarly competitive, I would imagine similar importance being given to GCSEs.
For AS take subjects you enjoy as long as they are 'traditional'. Again, aim for the best grades you can achieve, as they will be the basis for your A2 predictions. Also, if you decide to change your mind and apply to Cambridge, they ask for your AS module scores and will take them into account.
In your AS year, it will be equally important to build up a knowledge of law and an enthusiasm for it through reading around your subject. Perhaps start with some general introductory texts to introduce yourself to the subject then find a topic(s) that interests you and read about that as well. In addition to this, as you suggested getting work experience and attending law courts would be a good idea. Also, if your school or college has a debating society, it would be good idea to join to develop your argumentative skills. All of these things will help you express enthusiasm and knowledge of the subject in your personal statement and interviews at Oxford.
Sorry if I have misinformed you in any way here, but I feel that doing these things should boost your chances of getting a place on one of the most competitive courses. Also, don't get too focused on Oxford, with a subject like law there are many great universities out there that offer law courses. For example, UCL gives you the opportunity to go to Columbia University after your second year and graduate with both an LLB and JD.