Welcome to allnurses!
You would have to attend an MSN program that offers a concentration (major) in midwifery. There are plenty of them out there! Different schools have different requirements about how much, if any, clinical experience they require of applicants, so that would depend on the specific school.
I encourage you to review some of the older threads on this forum -- there is a lot of good info, and answers to questions you may have are probably posted here already.