I've got a degree in Kinesiology, so I often get asked questions about nutrition and diet (about exercise, too). Unless you are an athlete, have certain types of medical problems, food allergies, or major dietary restrictions (such as being vegan), a good diet is actually pretty easy. Note that I said "good", not "great" and not "perfect". One of the biggest mistakes people make when trying to improve their diets is to try to eat "perfectly", which is a foolish, counter-productive goal. Make your goal to just improve your diet, so that the changes you make are ones you can stick with. A "good" diet you can stick with is much better than the "perfect" diet that you can't stick with. Only make changes you can stay with for years, not just weeks or months.
Step 1: Limit yourself to one soda or other sugary drink per day. This change alone can make a huge difference for many people. It can easily reduce your calorie intake by 200-500 calories per day. But besides just the calories, sugary drinks are absorbed into your bloodstream very quickly. If you are running a marathon, this is a good thing, but if you are just sitting in your office, your body will have to release a lot of insulin to deal with the excess glucose in your bloodstream. This generally gets stored as fat, but even worse is that by continually forcing your body to deal with extreme amounts of excess glucose, you will gradually wear out your insulin response and start down the long path to diabetes. Even better is to not have sugary drinks at all (except for special occasions), but don't try that unless you can stick with it long-term.
Step 2: Reduce the amount of processed carbohydrates you eat. Sugar, flour, and rice are all very efficient and cheap ways to get calories into your body. They are easy to digest and you can eat a lot of them without feeling very full. They have very little in the way of nutrients, though, so you will either need to eat even more calories to get those nutrients, or become malnourished. Honey is better than sugar, whole wheat is better than white flour, and brown rice is better than white rice, because they have more nutrients and take longer to digest (which means you won't eat as much of them). But a whole wheat muffin dipped in honey is still a lot of calories for not much nutrition. Also, a lot of things labeled "whole wheat" hardly justify the label--what you want here are things with whole grains, sprouted grains, etc. (i.e., all the stuff you hated as a kid). A tactic I've used is to not eat sugar, bread, pasta, rice, etc. during the week (or at least try to minimize the amount you eat). I then eat all I want on the weekend, so I don't feel deprived of the sweet stuff that I love. While this isn't optimal, it is something I can stick with year in and year out, which makes a huge difference in my overall diet.
Step 3: Focus on eating high-quality, high-nutrient foods. This means making the majority of what you eat non-processed or lightly processed. Every time food is processed in almost any way, it loses nutrients and therefore is not as good for you as fresher food. There are some slight exceptions (frozen fruits and vegetables lose nutrients much slower than canned ones), but you are better off making fresh meats, fruits, vegetables, and nuts the staples of your diet. Processed meats and cheeses are not as good for you as unprocessed foods, but they generally are better than processed carbohydrates. Personally, I eat a decent amount of cured meats and cheeses, but I try to get really high-quality stuff so that I don't need a lot to be really satisfied.
If you do those three things (plus drink plenty of liquids, preferably water), you'll be in pretty good shape. You'll get the protein you need, along with other nutrients, and you'll do it without eating too many calories or feeling deprived.
You didn't say anything about eating less fat. I should avoid fat, right? Not really. Most people associate eating fat with getting fat, but processed carbohydrates are the real danger. Fats from high quality meats, nuts, etc. are generally good for you. Just avoid highly processed fats as much as you can (lard, salad dressings, rich sauces, etc.). Don't worry so much about fat vs. protein vs. carbohydrate--worry about processed vs. unprocessed.
I read a report that X was bad for you. It might even cause cancer. What should I do? Ignore it. Unless you have a specific medical problem or food allergy, just focusing on eating less processed food is what will make the biggest difference. Almost all reports about food being bad for you boil down to "highly processed foods are bad for you" or "a study found a small correlation between X and cancer". The first one you already know what to do about. The second one is just overblown reporting by people who do not understand science or statistics. Correlation is not causation and studies are very prone to this type of effect. If a study doesn't find a link that at least doubles the expected incidence, then ignore it. If a study does find a really strong correlation, you should still ignore it. If at least three independent, well-constructed studies all show a really strong correlation, then you might want to consider paying attention.
I heard that the all-grapefruit diet (or some other very restrictive diet) really works. What do you think? Any really restrictive diet will cause you to lose weight. The candy cane only diet is very effective, as is the pork rinds only diet. This is because you will eat less calories (if you can somehow stick to the diet). No one can really force themselves to eat just 2000 calories of candy every day and nothing else. These diets are terrible for you in the long run, but they will "work" while you are on them. Perhaps just cutting out sodas might be a better plan, though.
Some friends of mine tried this special diet (South Beach diet, Atkin's diet, Caveman diet, etc.) and it really worked. Shouldn't I try it too? Sure. Almost any diet like that will work because it keeps you from eating lots of processed carbohydrates. If you find one you can stick with or that otherwise appeals to you, go for it. Just make sure the diet has you eat a variety of high-nutrient fresh food. But take claims that a particular diet has some magical secret to good nutrition with a grain of salt. (By the way, salt, especially sea salt, is not really bad for you. Heavily processed foods with lots salt are bad for you, but the salt is not the primary problem.)
Surely there must be some simple secret about good nutrition you aren't telling me. What is it? Okay, you're right. There is a secret. Actually, there are two. Something else to keep in mind: if you eat right, take care of your body, and never indulge in delicious food that is bad for you--you'll still die anyway. Life is to be experienced, not remembered or solved.