Remember that this is just a tactic to see how you can act and speak under pressure. Dress professionally in a suit or similar conservative attire for both men and women. If you are a strong candidate, note that the medical school will be trying to sell itself to you as well. Medical schools want to accept candidates who they think will choose their school over others. After the interview, send thank you notes reinforcing your interest.
Most schools require at least three letters of recommendation.
Try to get at least one, if not two, from science instructors. Physicians who have supervised you for volunteer or research work are another good choice. If you have had other professors or supervisors who know you well and can attest to your work and character, go with those as well. It is important to select your writers carefully. Choose those people who can assess you favorably.