D.W.
Generally you have to have an undergraduate degree in Nutrition and then be licensed by the state as a Registered Dietician.
You should look at the curriculum at various local colleges/universities that offer Nutrition degrees and see what's required.
Anatomy and Physiology courses vary widely based upon the program. I majored in Biology in college, and I took an A&P class my senior year that almost broke me. It was designed for 1st year med students and had only a midterm and final to grade you. I'm sure, based upon the number of Registered Dieticians in the workforce, the class required is challenging but not ridiculously hard.
Good luck!