I am not sure I would call them requirements, but you must be Catholic and single. That's about it for pre-requisites. Then you decide what order you want to belong to. They all have different charisms. Some are more strict than others. You go through a period called postulancy in which you petition the order to become one of its members. Then you go through novitiate, a training period. At the end of novitiate if you still wish to join the order and the hierarchy of the order feels you would fit in you take vows of profession which usually involve promising poverty, chastity and obedience.