For the whole time up until 6th form I thought I wanted to do geography, I then started my ASs and realised that psychology was my thing, this then shaped my out-of-syllabus reading and which unis I looked at. Oxford may be good, but it's not the best for every subject. So take some time to realise which subject you truly want to be studying. Also, go visit the uni at some point as you get a proper 'feel' for a place when you go visit that you may not get from researching about the place. Remember that some unis aren't for everyone and this is still the case with oxford and cambridge even with their reputation.
Once you know exactly what course you want to be doing, then read around the subject as much as you can - immerse yourself in the subject and become a geek for the subject as it's your thing and will probably dictate how your life will pan out in the future.
I was also one of those people that thought that oxford was the be-all and end-all, but after I failed my ASs I gave up on the hope of getting in and didn't bother, so yes, it is a little early. But it's always good to have direction in your life and to have a 'prize' for you to achieve.