If you don't have a testimony of Joseph Smith, the Book of Mormon, or that the LDS church is Christ's church, then you shouldn't serve a mission until you gain a testimony of these. As a return missionary, I would prefer to have never dealt with "baby-sitting" missionaries that don't understand what "going on a mission" means. It would have made my time more productive if I had only had companions who were ready to obediently serve "with all their heart, might, mind, and strength."
I am not discouraging people from serving missions, just do what is necessary to have a strong testimony, and prepare yourself to work. It isn't a vacation, but is one of the most enjoyable work and lifestyles you will ever experience. It is life changing, and one of the best decisions I ever made!
