I would start with a class or a private teacher in the beginning just to get a grasp of the major grammar points of gender, case, etc.. If you've never studied another language that has a case system, learning German can be really confusing. On the other hand, if you've studied Latin, the German case system will be a piece of cake. After you get the basics, a language partner would help a lot. There's a lot of plain old memory involved as well, unless you are just aiming to make yourself understood and not worried too much about being grammatically correct.