I'm French and I want to study un UK

Please help me... your English system is not like ours at all. Who can help me to understand it? How to choose my courses at Oxford for example ? How to choose my college ? Please give me an answer if you can help me

What subject do you want to study (or, if you don't know, what subjects are you interested in)?

I am interested in law and politics but nothing is very sure for the moment ... I want to apply for 2018-2019

Is it a good idea to study in England with Brexit on the way? Tuition fees are already ridiculously high, and you don't want to be charged international fees halfway through your course.

Hey! I'm French and I study PPE (Philosophy, Politics and Economics) at Oriel college (at Oxford). I understand you completely, no one in my family had even thought of studying in the UK before I chose to, so I'd be happy to help!
If you're interested in law or politics, then you should look into Law courses first. I think some colleges in Oxford have Law and European Law courses in which you could do British and European Law (and not cut yourself completely from the French system). But those are very competitive. Then, you could look into PPE or History and Politics. You might like either of the two, but PPE can be challenging if you didn't do S (or at least ES), because of the maths involved in Economics courses.
Finally, don't worry too much about Brexit yet. Unis in the UK will try as hard as possible to stay attractive to European and International students, especially top ones like Oxford. And even if it indeed turns out to be a 'hard Brexit', I still think that some deals allowing students to move around and study anywhere in Europe will stay.