Your grades are an excellent start, so now you have to focus on the other components of the application. There are three main parts to consider before the (in)famous Oxford interview.

1) The Personal Statement and the Reference

You may have limited control over what your referee writes but it's important that these two pieces compliment each other. Work on your statement with whoever you ask to be your referee so that they know what it is exactly that you are trying to tell the admissions tutors and which achievements you have been forced to drop from your statement because of a lack of space.

In terms of your question on what to put into the statement, first and foremost, it needs a clear, precise and non-generic message. By this I mean you need a reason that has made you apply to Oxford for this degree that shows you have reflected on what you have done and read. The achievements you include in the statement should reinforce this message, show your commitment to exploring the subjects and if possible track the development of your interest to this point where you have an intellectual opinion on these two areas of study. In my statement I began with an introduction on how my upbringing in an international environment inspired and interest in the past and present of the places I lived, in the middle I had all the things I had done and I finished with a reflection on how I think history and politics are intrinsically linked. As a joint-schools applicant you do need to focus on the overlap between the two subjects. Think about this in relation to everything you have done, Oxford is looking for people who reflect on what they do! Bear in mind that whatever you write in your statement may come up in interview. Also it would be important to get some work experience in one of the subjects and mention it in the statement.

2) The Aptitude Test(s)

All applicants for History and joint-schools have to sit the History Aptitude Test (HAT). Past papers are available on the faculty website, use them to practice, everyone committed to making a competitive application does at least a few before the real thing in November. Try do them in the time allocated and ask your teacher to correct them, practice makes perfect (or at least good enough to be invited for interview)!

I believe there is also an aptitude test for Modern Languages applicants (the MLAT I think). I'm not sure how this one works but if you can get your hands on past papers do them too.

3) The Essay

For History you need to submit an essay done as part of your school work. Keep this in mind whenever you write essays this year. Again this is something that can come up in interview, so choose something you have a lot of background knowledge in

Admission to Oxford University as an international student at the undergraduate level is tough.
Really TOUGH.

You can apply if you manage to secure a sub-2000 rank in JEE- Advanced.

After all, they really want the best of the best!!