You did not take the IB diploma? You may still qualify: Although the US high school diploma is not consider equal to A-levels or IB, for some universities you may present alternatives for admission purposes:

At least a score of 550 in the critical reading, math, and writing sections of the SAT Reasoning Test and at least 550 on any relevant SAT Subject Tests, OR
Grades of 3 and above in at least two, and preferably three, AP exams.
Selective universities and high demand courses will have more stringent requirements than the above, and students should contact the departmental admissions tutors to find out about specific additional requirements. In general, students will be expected to submit AP and SAT Subject Test scores in the field of proposed study. For example, students interested in Biology should present AP and SAT Subject Test scores in Biology, Chemistry, and Math. Since admission decisions are made by departmental admission tutors and not by the overall university, having exams in the appropriate subjects is critical. Without these exams, students would be unlikely to receive an offer.

In addition to reviewing exam scores, the decision to offer admission is also based upon whether the student has demonstrated interest in the course of study applied for. Taking classes during the academic year, doing independent reading, joining related extracurricular clubs, and participating in summer experiences related to the course of study are all suitable ways to demonstrate your interest.

Course grades are of less interest to university entrance tutors, but can be included on the UCAS form if they further demonstrate academic strength in the subject applied for. Talk with your counselor about how to handle transcript information in your particular situation.

The student will be asked to write a personal statement as part of the UCAS application form. These statements are far more specific than essays written for U.S. colleges, and concentrate on reasons for choice of the specific degree, experience and skills related to the choice, career aspirations, and a short paragraph about contributions to school and community life. For students living outside the UK, a brief explanation of the reasons for studying in the UK should be included.

A one-page reference written by a teacher or the student's counselor forms the final portion of the UCAS application. The reference primarily addresses the student's suitability and ability to be successful in the proposed course of study. It will also discuss the students' personal qualities and accomplishments and should make predictions about the students' scores on any Grade 12 AP exams. Finally, the reference will explain anything about our school or the students' particular situation that would help admissions tutors put the test results and academic background into context.

Students who have not taken AP courses, or who have taken only a small number, may be better off applying for a one-year foundation course prior to applying for a university degree course. Since many UK degrees can be completed in three years, taking a foundation course will not necessarily mean longer than four years of university study.