I guess what I'm trying to ask is, how do I narrow down exactly what I want to write about? I've been interested in health and wellness ever since I was a teenager, and now I'm in my mid twenties. I feel like I have a lot to share about health and fitness, but I don't think it's information that others haven't already mentioned over the web. I don't know if it's even worth it at this point to write one considering that fact. Please help give me guidance.

Start writing. Do you have a focal point? -- vitamins, certain foods, exercise, yoga, anecdotes?

Write down everything you want to share. Give it a title. You have a book.

Well I can but am not sure how scientific I want to get with the diet information. But I do have both diet and exercise ideas to implement. I want to insert sample workout routines for an idea, but also give the reader the ability to take the concept and apply it to their own regimens. And yes, yoga + stretching is included for the recovery of weight lifting.

You will never find (or seldom find) something not already out on the web. Some of the classic books "Think and Grow Rich" for example, has the same material available everywhere, and even 100's of new books with about the same information.

Books get read and sold, not always because they present new information, but that fact they are marketed well.

I can recommend 2 ways how to begin. First one is you sit and start writing those things you want to tell about in your book. For example, it is great if you tell about your cooking or work out routine on your own example. As soon as you wrote something, you will understand which parts you missing, the most important is to begin.
And the second way - make a plan of everything you want to tell about, as precise as possible. And then start adding volume to each part of it.