1. Buy and eat real food. With that I mean that real food doesn't need to be packaged, such as fruit and veggies. I know that those can be packaged too, but you know what I mean :). Even if you have to eat cookies, try to make them yourself, because you'll have to "work" for them, you probably won't be baking and eating them every day.
2. Packaged and canned food is allowed too, but it's good to read labels. Packaged food that is good for you: quinoa, amaranth, brown rice, whole grain pasta, canned beans, tuna, white cheese, Greek yogurt ... Those are mainly one to three ingredient foods, and I kinda "allow" myself five ingredients, but I always check if there are all ingredients that I can pronounce. If I don't know the ingredient, I won't buy it.
3. Another good tip is to ask yourself: Is this food good for me? Will it benefit me? Is it healthy? Does my body really need this? If the answer is no, than don't buy/eat it. Avarage person eats too many empty calories per day already. I'll write about empty calories in the future.
And don't get me wrong - having a cheat meal every once in a while is ok, it's not ok to have cheat meal life :)
I hope you find this post somewhat helpful. :)