You could go to the gym every day, workout like a madman and gain strength through the weight training. If your diet is not consistent with your routine, you won't look too much different. That muscle will grow, but the fat will stay over top of the muscle, making you look no different. You need a diet that will give you the essential elements to build muscle, but eliminate the bad ones that lead to excess fat.
The first thing that you'll need to know and understand is that what it all comes down to is regimenting. This is a lifestyle change and those can be hard. That is why most people gain the weight back that they lose, because they don't integrate the lifestyle change permanently into their lives. Get into an eating routine, and stick to it even when you're not seeing results. You will.
Your protein intake should be 1 gram per every pound of body weight per day. You can best get this naturally or from a pure protein powder. Isolates will work best for those individuals who are overweight, because they are pure and contain fewer calories.
The way you will lose calories is start by eating 18 x your body weight. So if you are 200 lbs, eat 3600 calories per day. Then wait a week. Weight yourself and find out if you lost any weight. If you didn't cut back 500 calories and wait out the week again. Do this until you find you are losing a healthy amount of weight. Also note that bodybuilding will increase your weight through the muscle development and muscle weighs a bit more than fat.
Balance your 3 types of fat intake. Omega 3, 6 & 9 fats should be mixed up and taken in on an even scale.
Load on the vegetables (especially dark green vegetables). Do the same with fruit. You should be drinking 1 liter of water per every 1,000 calories you expend during the day.
Always eat foods that come in their natural state. No processed foods. Avoid foods that come out of a box. Avoid trans-fats. Give yourself one day a week to eat a limited amount of junk food and stick to stopping when you've reached that limit. If you can't stop yourself, you'll need to cut out the junk food entirely.
The last thing you'll want to do is starve yourself. If your body goes into starvation mode and it needs to burn calories for energy, it'll head for the muscle first, not the fat. So you're shooting yourself in the foot by doing this.
You won't see gains right away, it's going to take some time and patience to see what you are working so hard for. But have a little patience and you will see a difference in the way you look. Continue to measure your waist, weigh yourself and even go in for blood work every once in a while to keep track of your progress so you know you are making a difference. You'll get there.