As everyone else has said its about eating the right types of food. Processed crappy foods are going to make you fat, plus they are very high in sodium (salt) which isn't good for you, lunch meat is a big one. Eat lean meats (chicken,fish,etc),fruits and lots of veggies. Foods with fiber are good because they will fill you up and help with digestion. Protein is good too, it fills you up and I believe is digested slower which helps maintain good blood sugar levels (Even if you aren't diabetic you can still feel funny from low blood sugar, if you haven't eaten,etc).
I get around 100g of protein a day plus 25 grams of fiber (men need more fiber than women, I think at least 35 grams is recommended). It's also about burning more calories than you take in. Its not hard, its all about making the right choices.
Also don't cut out foods completely. I still eat things like chocolate but in moderation. I usually have a square of dark chocolate after dinner which helps my sweet cravings. Also once a week I have a cheat meal, where I'll eat something I want, pizza, for example. If you totally deprive yourself of things you will go crazy. I don't feel quilty about it because I know Ive done well earlier in the week, and a few slices of pizza won't kill me.
Calories are not the created equal. Eating 2,000 calories of healthy, balanced foods is best. Eating 2,000 calories worth of cheeseburgers, oreos, ice cream, and soda will make you fat.
Drink more water, it also fills you up. If you feel hungry but you just ate a few hours ago, try and drink some water and see if your still hungry. Eating more fiber will keep you fuller. Eating slowly will allow the trigger receptors in your brain to signal that your fuller sooner. If your inhale your meal you'll most likely feel stuffed. Which could be because of how fast you ate, the air you took in by eating too fast,etc.
If you feel you eat too much sodium, which is easy today since its in everything, eat more pottassium as it filters out sodium. Its basic biology. Most people probably get at least 2,000 mg of sodium which is over the recommended daily limit. Limiting high processed foods like cheese and lunch meat (which packs a lot of sodium) is a good start.
Another thing to remember is that fat in foods won't make you fat. Its overeating calories that will make you gain weight. I get a good amount of fat in my diet but its from healthy fats, olive oil,nuts.