Most people want to lose at least a little weight. Hopefully this will be helpful to someone.
Before saying anything, it's important to point out - CHANGING YOUR DIET SHOULD NOT BE A TEMPORARY THING IF YOU WANT LONG TERM RESULTS. YOU SHOULD THINK OF IT AS A LIFE-LONG COMMITMENT, NOT A DIET.
The Basics - At the most basic level, weight is simply Calories in-Calories Out. That is, more calories = more weight. So in order to lower your weight, you will want to reduce your calories per day overall. It's not simple enough to stop this post there though, so lets get started.
What *are* Calories? If you just look at the nutrition information for any food it doesn't give you much of an idea. Where all the normal parts of the label are broken down into different components (Fat, Protein, Carbs, Sodium etc...) Calories are the opposite. Calories measure how much energy is released when a food is burned, which means it takes into account everything else that is in the food - It's the sum of everything in the food as far as energy goes.
That means that first of all, calories aren't bad. You need calories in order to live. Most people are just taking in too many of them. So what's the best way to reduce your calories? You can either lower your caloric intake or burn more calories through exercising. Unless you're already very careful with what you eat, diet is the best way to reduce your calories. This is simply because (as anyone that's ever watched the calorie meter on a cardio machine knows) taking in less calories is far easier than burning them off. Of course, doing both is certainly optimal.
Before I explain what to look for in food, it's important to note why you have to *change* what you eat and not just stop eating. The first reason should be fairly obvious, but it isn't for most people. When you starve yourself you do lose weight, but the question is where is that weight coming from? While some of the weight is certainly fat, a lot of the weight loss is also muscle loss. Doing that completely goes against your self interest as muscle isn't what is making you 'Look bad,' and more importantly, because the more muscle you have the more calories your body burns at rest - while doing absolutely nothing, making it that much harder to keep weight off in the long run. It just leads to more starving, a bad cycle.
The second reason you can't just stop eating food is because of what's commonly referred to as "Starvation Mode." When your body realizes that it's not getting many calories it goes into Starvation Mode. Basically what that means is it hoards as much fat as possible, and it lowers your metabolism - lowering your energy. Evolutionarily speaking, this makes total sense. In the infancy of our species food wasn't readily available like it is now. So if someone were to go an extended period of time without food then keeping the metabolism high, burning lots of calories in the process, would be a very bad thing for survival. So by starving yourself you lose lots of energy which in itself sucks, but it also makes exercising that much more difficult than it already is, and making it that much harder to lose weight in the long run. Of course, your body hoarding fat also once again is completely counter productive to your actual goal.
"That makes a lot of sense Aaron! But why fat? Why doesn't the body turn all those extra calories into muscle and make me a monster!?" Biology my friend :)
I'm sure you've all heard the expression, "Muscle weighs more than fat." That's actually is completely true. Chemically Fat and Ethanol hold the most energy per mass. Since Ethanol is alcohol, hoarding it would make survival...difficult. Turning all the extra calories into muscle, while in this day in age would be incredibly cool, would be suicide for the human species before food was so readily available. It would weigh us down far more, making it harder to catch prey while containing far less energy for us to live off of if food became scarce.
That brings us back to eating. Since we can't starve ourselves to lose weight, how do we make sure the calories we're taking in don't turn to fat? As I said earlier, at the basic level it's calorie in-calorie out. If you burn more calories than you take in every day, you'll lose weight. Losing weight isn't the true goal though, losing fat is, and that's where it gets more complicated. As I also stated earlier, calories are a sum of everything else in the food. So the goal is to eat foods that have the most beneficial calories. That's why people say things like soda and bread are 'empty' calories. They provide no beneficial nutrients, and it's been suggested that soda might actually deplete your bodies nutrients.
To make the most of your calories you have to understand what constitutes the majority of them. The four biggest sources of calories are Sugar, Fat, Carbohydrates, and Protein. Sugar contributes the least to calories of the four, but health-wise it's terrible for you so it's the best thing to cut first. The less sugar the better. That's pretty common knowledge, and after that is where most people have been misinformed.
Neither Carbohydrates nor fat are universally bad. Our bodies need both to survive. The simplified diagram of what to look for out of your calories is Protein > Carbohydrates > fat > sugar. It's not exactly that simple though. Good fats will be far better for weight loss than bad carbs and vice versa with good carbs vs. bad fats. As far as fats go, saturated fats and trans fats should be avoided whenever possible, trans fats being the worst. Unsaturated fats are good fats. EFA's (Essential Fatty Acids) fall in this group. EFA's can't be made by the body so must be gained from food. Fish are a great source for them as well as many seeds and oils. Bad carbohydrates are things like white rice, white bread, and other high starch foods. Good carbs are from fruits, vegetables, and whole grains (brown rice/bread etc..)
Protein is the best thing to get calories from. In most cases, carbohydrates will make up the majority of your calories simply because they're so abundant in food, but the higher the protein in relation to them the better. Protein takes more energy to break down than the other components which gives it an advantage right from the start. Your body uses more energy (therefore more calories) to break it down than it would to break down the other things. Protein is also very important for rebuilding muscles which makes eating right in combination with exercising that much more powerful. Because protein is harder to break down it also stays in your stomach longer, making you feel fuller longer and want to eat less.
So the complicated version is Protein > Good Fats > Good Carbs > Bad Carbs > Bad Fats > Sugar.
Talking purely in calories still leaves off one major component though: Fiber. Ever wonder why high fiber foods have such low calories? The reason is because our bodies can't break down fiber. If our body can't break it down, it can't get calories from it. Fiber is great for weight loss for a couple of reasons. 1. It fills you up without adding calories. 2. There are two kinds of fiber - Soluble and Non-Soluble. Non-Soluble fiber helps with your bowl movements by pushing things along with it as it goes through the intestines. That's why people say to eat more fiber if you're constipated. Soluble fiber when dissolved actually binds to some fats, which then don't get digested and instead go right through with the fiber. So it not only fills you up, but takes some fat away at the same time.
So fiber is great with a normal diet. Since it can't be digested though, you can't get any nutrients from it. That's why vegans and vegetarians have to be careful to make sure they don't become vitamin or nutrient deficient.
NOTE - HIGH PROTEIN IN A FOOD DOESN'T NECESSARILY MAKE IT A GOOD THING TO EAT. IT'S THE RATIO OF PROTEIN TO THE OTHER COMPONENTS THAT MATTERS.
Note - If you have Diabetes, are severely overweight, or have a genetic disposition towards Diabetes (If you have closely-related family members with it) then you should be careful with taking in high quantities of protein as its hard on the kidneys. A high GOOD carb and GOOD fat diet will probably better for your overall health if this is the case.
Note - High amounts of protein will cause your calcium to be lower. It might be a good idea to include calcium in your diet as well, or to take a calcium supplement if you want to go the high-protein route.