The basic level of Registered Nurse is a starting point (not actually, but I will get to that in a minute). You can test for your nursing license with an Associate Degree from a community college, or a Bachelors Degree from a 4 year college or university. With a RN license, you are eligable to work at a psych unit. There is also a certification in psychiatric nursing from an accrediting agency, the ANCC (American Nurses Credentialing Center). Having this credential seldom really gives one an advantage, in terms of more pay, better job, etc. It does reflect a level on knowledge of the area of practice, a sort of bragging rights.
There is an interesting career pathway that I took. Start out working as a tech or nursing assistant on a psych unit. This will give you great experience, and some hospitals will even help pay for your education to become a nurse. I completed my 2 year degree first, and got a job working as a nurse on the psych unit I had been a tech on. I went on to get my 4 year degree while working there, and am now completing my degree as a psychiatric nurse practitioner, many years later. The level of education as a psych nurse can range from training as a CNA to a masters degree/nurse practitioner, but all in the field of psych.