The most important thing is to decide what subject(s) you want to study (which will probably involve thinking about potential careers - make use of all the resources available to you: the internet, careers advisers, careers' libraries etc.). Having done that you can then look at which universities offer the best courses in those subject areas (the UCAS website is a good starting point). Now you should be in a position to draw up a short list of potential universites, based on the factors important to you (maybe campus/non campus, location etc). Make use of the univerities' websites and that of their students' unions, to learn more about them and their ethos. Then go along and visit these places - you need to visit a city you're going to be living in for three or four years. Go with an open mind, take notebook and pen to note down what you see and hear. What do you think of the atmosphere? Are current students/tutors approachable and friendly? If there are particular extra-curricular activities important to you, what's the provision for them like? What's your gut feeling? Could you live and study here for three years? You should now have isolated your six choices and be in a position to write your application, so do it, and submit it as soon as you can. The earlier you submit it the earlier it gets to the universities and this increases your chances of getting offers.
If you're specifically thinking of applying to Oxford all of the above advice applies, but remember that Oxford is a collegiate university - as well as looking at the course content and university wide information, you should have a good look at the different colleges. College choice isn't as important as it might seem when you apply; everyone ends up being happy and enjoying their own college, but there are some things to consider. Firstly, look at battels levels (college accommodation and other charges); it is significantly more expensive to go to some colleges than others. The OUSU website has a survey of battels levels, but be careful when you compare them; some colleges include a certain number of meals or a photocopying allowance within their battels making it quite difficult to make a direct comparison between them. Then think about location - where do you want to be within the city? Where are your lectures going to be? (this is particularly important if you're a scientist or mathmatician). Also look at the application:places ratio (but don't put too much emphasis on this, since it varies widely from year to year).
Once you've had a good look at all the official information provided - university, faculty, college, OUSU websites, then take a look at Oxford Gossip for a different perspective - remember that this site is (probably) a fairly small subset of Oxford students (and a few former students) playing. It's not designed to be a resource for prospective students - but if you're capable of studying there you'll be able to deal with that. You can learn a lot from reading what's there, though I wouldn't advise posting questions there without reading through the kind of replies other prospective students have received:-) There are lots of 'official' places you can go where you'll get sensible replies to queries. I would, however, encourage you to join in some of the arguments - one of the reasons Oxford graduates are so valued in the workplace is that we can defend our arguments when we're challenged. See how well you get on when you put forward a view point and someone else challenges it; there's nothing shameful in revising your point of view to take account of new information, or in conceding defeat to someone who knows more about the subject, but give it a go - you've got nothing to lose!