Each of Michigan's 83 counties elects a Prosecuting Attorney every four years (during the Presidential election). Depending on staffing needs and budgets, each Prosecuting Attorney may hire Assistant Prosecuting Attorneys (APAs). Small counties may not have any APAs, while the largest counties may have several hundred to handle high caseloads and the wide-ranging responsibilities imposed on prosecutors by statute.
Prosecuting Attorneys and APAs are lawyers, licensed to practice in Michigan. As with other, prosecutors generally complete a four-year college degree and then go to law school, which generally takes three more years. After graduation, most states (including Michigan) require lawyers to pass an examination to become licensed to practice law in that state. Michigan's "bar exam" takes two days, but other states' exams take three days.
Job openings in Prosecuting Attorney offices occur periodically. If you are interested in working in a particular county, you should contact that Prosecuting Attorney for information on job openings, qualification requirements, etc.