To be brief you should focus on GPA, physical fitness, and leadership. Get involved with something where you can have a leadership position such as a student organization. The interview is like a normal job interview and you are expected to know information about the Air Force such as the core values and mission statement. During your application process there will be a write up you will complete that the interviewer will use to assess you so be genuine. You choose either OB or med/surg and if you choose OB you are basically stuck there forever except in rare cases. Hope this helps.