Soop:
Diet can vary a lot depending on what you're trying to accomplish (fat loss, muscle gain, etc). General rule of thumb is just portion control. I'd honestly say weight loss is 70% diet and 30% exercise in the initial phases. Lose weight with diet and use exercise to build muscle, tone up and feel better.
Lots of fruits, veggies and fiber help out a lot as well as limiting your types of meat and processed foods. If you're doing a lot of heavy weightlifting or playing sports, using a whey protein shake can be a good recovery tool to get you back in the gym or on the field sooner with less soreness.
If you're really serious about getting more muscle mass, Creatine does work very well but be warned that the results tend to disappear once you stop cycling through it.
Here are some general "lifestyle change" kinds of things that work for me:
1.) Limit bread intake of any kind (switch to wraps when possible)
2.) Get rid of chips/dips/bad snacks. Replace with unsalted almonds (in moderation), fruit, etc.
3.) Get in the habit of eating a piece of fruit / yogurt / glass of water in the morning. don't skip breakfast but don't eat a dozen donuts either
4.) Avoid red meat - do chicken or fish instead when possible
5.) Everything in moderation - diets are about a permanent lifestyle change, not about dumping everything you love for a set period of time. You'll go nuts if you don't have the occasional pizza, just be smart about it