Hi Katie -
There are a lot of home courses you can take these days that focus on holistic nutrition. Ultimately, you have to find one that you resonate with and also consider how much time/money you want to spend.
I can't think of one that I really recommend, because honestly I don't resonate with most of them. Either they take a very "limited" approach by focusing on one particular kind of diet (i.e. macrobiotic, vegetarian, etc.) or they focus a lot on isolated nutrients and their therapeutic application (i.e. taking vitamin supplements to "cure" a particular condition).
I got certified as a nutritional consultant through the American Assoc of Nutritional Consultants, which came the closest to what I was looking for for my purposes.
Think about what you want to do first - very specifically. Then look around for a program that will suit your needs.