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First you must become an RN with either an ADN (2 yrs + time to take prerequisites) or a BSN (4 yr program). Then, depending on which one you took, you will need to take an RN-MSN-CNM program which will take you an estimated 2-6 additional years on top of that.
A CNM is a nurse with a master's degree and advanced certification and affiliations and usually several years of experience in women's health. You might want to check out http://www.midwife.org/ for more information.

Of course there are midwives that aren't nurses, they are usually a lot less school and a lot more on-the-job clinical experience following around an experience midwife and delivery a certain amount of births. However, these women are not allowed to work in a hospital setting, nor can they prescribe medications, etc as they are not nurses. They are generally for women in rural areas or women who want to deliver at home and have a very routine low risk pregnancy.

Source(s):
OB nurse