What should I do?

Hey guys I need some help. I want to know what martial art is right for me. I want to be able to protect someone I am with. Now I realize that the best thing to do in situations like that is to run or call an authority or something but I am a firm believer in being prepared. I am 6 feet 1 inch tall and weigh about 180 pounds. I am in pretty good shape and have done several sports throughout my life. If you guys could give me a little direction that would be awesome. Thanks and have a nice day.