14 Yr old UK international applying to MITWatch
You apply to MIT directly, they provide all the information required on their website. You would apply at the same time you would normally apply for unis in the UK (in your last year of school, starting in the fall - so usually early in year 13), although some of the deadlines are slightly different. The major difference is you will need to take the SAT or ACT, which I would suggest aiming to do in one of the summer dates before starting your last year of school to give you time to retake it if you need to.
Note that US unis (including MIT) put a lot more weight on extracurricular activities than UK unis, and generally want to see you have engaged in a wide range of activities at a high level, consistently over a long(er) period. They also want to see you have demonstrated leadership qualities in those, usually. So, just turning up to debate club or something, even if it's over the course of 4 years or so, doesn't really flag on their radar. They would want to see that you have e.g. been the head of the debate club and used that position to make meaningful, long term change within the club and also done something notable with it (e.g. led your team to compete at a regional level at least). That is just an example of course and there are many other ECs you could do with different ways to excel in them. But they do want to see you excel.
Getting into "top" unis in the UK is perhaps comparatively more straightforward because they basically only care about how well you have done academically and in the specific subject area(s) you've applied to. So whether you play the cello at lead in a major symphony level is irrelevant if you aren't applying for music as far as Oxford or Cambridge are concerned, for example - they would only want to see what you have done notably in your subject area (e.g. if you were applying for maths, taking part in the senior maths challenge/BMO would be very promising, although that is by no means required anyway).
This is of course assuming you are looking to apply there later during your A-levels as normal. If you are asking if you can apply now before even taking your GCSEs the answer is no, because they require a high school diploma or equivalent to apply I believe (which you will not have - the equivalent in the UK being completing your A-level/equivalent studies).
The competition for a place is extremely high, so try and get into a International Olympiad for Maths, Physics, Chemistry or Bio etc. You can do this by doing good in UKMT intermediate challenges and Intermediate Math Olympiads to be invited to the international ones.
Next, a perfect SAT score and ACT (not completely sure though). Check their page for more clarification.
This is just what I know as a UK student as well.