Well, depends. What do you eat regularly? I also am very skeptical of anyone that doesn't really know much about health (not saying that's you, just in general), that tells me that they "feel good" and are "healthy". Because they usually aren't even close to it, and their definition of "feeling good" is more like "feeling average". I don't think people really have a feel for how you should feel, physically and mentally, as a healthy person. Granted, all of that is pretty hard to measure and whatnot, but whatever.
To me, the first step would be cutting out grains/gluten, dairy, and processed foods for 30 days. And then see how you feel. After the 30 days are over, I'd wait one more week, and then try to introduce grains slowly back into your diet. The point isn't to actually get them back in your diet (personally, I don't think there is a single person on this planet who can truly eat gluten without any issues, as the "grains" we eat today are not really grains), but to see how they make you feel.
For me, I stopped eating grains for 30 days to see what was up - because I didn't feel like grains were causing me any issues. When I tried to eat them again on day 35 or so, I would get headaches, my stomach would be insanely bloated, and I would feel like shit for the rest of the day - symptoms that I didn't have before. My theory on why this happened is that I had fucked up my system so bad that I couldn't really feel the damage it was doing. When I took it out and my system had time to actually repair, when I reintroduced it it was like "what the FUCK is this shit" and went haywire.
Anyway, I think that's a good place to start.