Every applicant to medical school, or any other professional or graduate program, must provide letters of recommendation to the Admissions Committee. It is key to your success that you think about letters of recommendation well ahead of time. You should not be asking professors or employers to write letters when you can only give them a short deadline.
Most medical schools look for three letters of recommendation.
Generally, this means that you should arrange for two letters from professors in the sciences and one from a non-science professor. It may be appropriate in some cases to ask an employer or someone else to write a letter for you if he or she is in the position to judge the quality of your work. Every professor from whom you take a class is a potential recommender for you. Don't just go to class - study hard, get good grades, and get to know your professors. Develop relationships with those at the University of Michigan whose letters will mean something to medical school admissions committees.