Depending on what you want to study in Oxford, there are specific subject requirements and the recommended level. You should look up the entry requirements for your chosen course at the university. You might have to take additional state exams or Advanced Placement exams if you are not doing A levels or IB. Generally, you should aim for the highest marks possible. 

Submit your application via UCAS, a platform that allows you to send one application to 5 universities in the United Kingdom. The main aspects of the application are your grades and academic preparation, alongside your personal statement where you share what activities you have done related to your course.