I'm not sure if what we call 'Oxbridge', the big 2 (Oxford & Cambridge) handle their own applications, but I'm pretty sure Ucas is the place you need to start.
I know it works (at least for UK students) on a points system, where you get points per course, and the amount is set by the grades you get.
Things are different if your a 'mature' student, and if you have 'life experience'. Best thing to do is get in touch, and write a very compelling 'personal statement'. The latter being the first step to application.
I'm sure theres a few guys here who can give you more (oxbridge) related advice.