For the past seven years, I've been very strict about my diet. My diet has gone through various incarnations, with most things besides meat consumption being relatively consistent over the past seven years. Throughout this time I've read a lot about diet, experimented a lot, and learned a lot.
While I think that my whole diet is close to the optimal way for humans to eat, I'll be the first to admit that the various changes I've made have had magnitudes of impact upon my health. If you're starting out transitioning to eating healthy, you might be better off following a subset of the diet to get most of the benefits with a fraction of the change. As with most things, consistency is more important than perfection.
By far the most important thing you can do for your health is eliminate all sweeteners from it. That means sugar, high fructose corn syrup, honey, molasses, aspartame, splenda, agave nectar, etc. If it is meant to be added to something to make it sweeter, don't eat it. This does not eliminate whole fruit, but does eliminate things sweetened with fruit juice. This sounds extreme at first, but it's actually not bad at all. Yes, it takes time to get used to not having things that are sweetened. As a result, your tastebuds recalibrate and eveything tastes sweeter. In my most strict phase, I went for around three years with no sweeteners at all. It really wasn't hard.
Eliminating sweeteners is 40% of having a good diet. That's a big win. The next twenty percent is refined grains. White flour is the worst of these, but white rice, pasta, and corn flour are also bad. The catch here is that every advertiser will try to trick you into thinking that what you're eating is whole grains, even when it's not. Whole wheat bread, for example, is almost never actually made from whole grains. Usually it's mostly white flour with a bit of whole wheat in it. Not good enough. Ezekiel and Alvarado St. Bakery are the two bread brands I'm aware of that are actually good.