Oxford and Cambridge universities are collectively known as Oxbridge, and are two of the most prestigious places of learning anywhere in the world. Demand for a place is enormous so take the following advice to ensure that you application has a chance to stand out from the crowd:
Applying to Oxbridge comes with a set of deadlines which are sooner than for other universities (your application must be sent by the 15th of October). When you add to that the requirement to sit entrance tests for some courses, and the amount of extra reading and preparation that is required to have a strong application, then you are looking to get serious about an Oxbridge application 18 months to two years before the date that you would start your course. That means working quite seriously on strengthening your application throughout your penultimate school year (S5/ Year 12/ Lower 6th Form). The summer before you enter your final year should be used productively to prepare for what can be a difficult process which is likely to include an interview.
It seems obvious, but you should make sure that you have checked the website and read up on the process involved in applying. You can only apply to Oxford or Cambridge, and not both, and you can choose to apply for a specific college or to submit an open application for any college in the University that will take you. You should also be really clear in your own mind which course it is that you want to apply for, you might well be torn between two or need time to make a final decision, but deciding which course you are applying for should be more important to you than the college or university you go to, as you will be completely immersed in this for at least the next three years, as well as gearing your preparation towards this.
The super-curriculum is for everyone regardless of university choice and even simply for the love of learning, but if you are applying to Oxbridge then you should be engaging heavily in these pursuits. You don't need to have Oxbridge levels of knowledge of the course that you wish to study, they are going to teach you that, but you should have a strong and demonstrable record of your interest in that field and also enough knowledge that you could comfortably discuss aspects of it in an interview. The course, college or university pages may well have information on possible reading lists or preparation that would be useful, and that is a good starting point; however, it pays to have a diverse set of experiences, and reading lots of books or articles may not be as useful as reading a more select few alongside watching several lectures, entering a competition and organising a school event, which may take up the same amount of time in total, but is far more diverse. Entering competitions in particular can be very helpful as it forces you to engage at a high level with your subject matter. The subject areas on this website have advice as to what you could do. Ideally you would start that process early in your school career; it is an important part of your application, but it is also an ongoing process and part of getting yourself ready for actually studying at that level. The Super-Curriculum should form a central component of your Oxbridge preparation.
A good application will have extensive super-curricular evidence, but that will catch you out if you haven't actually engaged with it properly. At an interview, you could be asked about any aspect of your application; they are not there to catch you out, they want to get to know you and learn about your potential, but if you profess a love of this field and an interest in a certain author or academic's work, then you really should make sure that you can discuss that confidently, or it will undermine the rest of your application. Be honest, and if you think that reading a certain book, undertaking an activity or delivering a school event would boost your profile, then make sure that you actually do it. Remember too that getting into Oxbridge is one thing, but you have to be prepared for being there too, so these things are not tasks to be ticked off on your way to getting in, but part of the well-rounded preparation required to work successfully at that level of study.
The people reading your personal statement are experts in the field that you are applying to, and are unlikely to require a definition of Economics. They have also seen hundreds of applications, and are not likely to be impressed by a quote you have found online, or by overly contrived introductions. Instead, they want to see that you are interested in the subject, that you have carried out extensive super-curricular activities and to find out about your skills and suitability for the course. Don't waste too many words talking about things that are not about you, your achievements and experiences, or your suitability for study, but by all means link your experiences and super-curricular work to the course and expand on some of the things that you have been reading about, learning or doing. One or two intelligent insights into what you have been reading or watching is far more engaging than a whole statement taken up with other people's quotes, ideas or theories.