Well for Law, it's most important to just do subjects that you're interested in - I got in to Cambridge with Physics, Chemistry & Geography, hardly traditional subjects for law! Indeed, around 50% of offer holders come from vaguely scientific backgrounds. However, I do very much regret not taking Philosophy & Ethics as it would have been incredibly helpful, and prepares you well for interviews and thinking about ethical and moral problems analytically, logically and quickly, as required in interviews/LNAT/Cambridge law test. If your school offers it, I'd recommend Philosophy & Ethics instead of straight Philosophy (which few schools actually offer anyway).
Other than that, just steer well clear of English Language, it's not considered a very rigorous course and is less favoured to various extents by different oxbridge admissions tutors - Trinity, Cambridge especially take a dim view of it. Lit/Lang is ok, but Eng Lit would be best. GCSEs absolutely fine for Cambridge - both will view in context of education, but Oxford puts more emphasis on them for interview pre-selection (which cambridge don't even do), so bear that in mind. That said, you'll KNOW which one you'll feel more at home at by visiting both, they ahve very different feels and most people don't struggle to choose.
More importantly, you need to read a lot and talk about it in your personal statement - extracurriculars, other than work experience, really won't help you too much. It's recommended to try and read a good newspaper daily (Times, Telegraph, Guardian or Independent) as well as a news magazine (Spectator or Economist) and to keenly follow current affairs - in my interview, for example, I was asked who the Justice Secretary and Chancellor of the Exchequer are, as well as my views on gun controls in america etc.
Also, read these two books: 'What about Law? 2nd Edition' by Graham Virgo and others, and 'Letters to a Law Student' by Nick McBride. These will also hopefully help you make sure law is what you want to do. This time next year, perhaps also try reading a few of the extended reading books they suggest, which will give you something different to put in your personal statement (most people will include the books I just mentioned, so don't bother putting them in - but VERY well worth reading). Also, before you take your LNAT (Law National Admissions Test), make sure you do both of the practice papers on the website and a few of the practice essays, too. If you end up applying to Cambridge, you may be requested to sit the Cambridge Law Test, of which there are 3 'flavours' - try and find out which one in advance (they don't have to tell you!) and again, do the practices for that.
There, that's my best advice based on personal experience! I've probably missed something out, and this is WAY off for you yet remember, but feel free to PM/reply if you've got any more questions