Study hypnosis. Hypnosis is the basis for performing past-life regression. You can study it through books and videos or you can take specific classes in hypnosis and practice it at home with friends or family.
Visit a hypnotist. Experiment with different hypnotists until you find one who works well for you and whom you feel a sense of trust with. Ask him to perform a past-life regression on you. No medical internships exists in PLR therapy, so you are best served finding someone yourself who is good at it and learning from him.
Ask the hypnotist if he can either teach you the process of PLR or refer you to a teacher or school where he learned how to do it. There are a number of different schools and programs out there for hypnosis and PLR and he can help you locate one that can work for you if he is unable to teach you himself.