This might sound funny but I actually heard a commercial on tv the other day for this...it was for Penn Foster school...its an online school. They have a fitness trainer and nutrition program. I actually looked it up online cause I was always curious about how to become one too. Obviously, we have to experiance training/nutrition to know about it...but this gives you a diploma and then you can take the national test after you graduate. Go to:
www.FitnessNutrition.PennFoster.edu Hopefully this will help you or atleast lead you in the right direction!