I'm trying to reduce my belly fat, for which I'm doing exercise for a month now, but no result yet. So now thinking of starting diet. But if I do that, and eat much less calories, then what will be burnt or reduced first, muscles or fat. Because I have earned nice muscles after a lot of hard work, I don't want to loose them. So I'm afraid that, if I take too much less calories then what will happen, either my fat is reduced(burnt) first or both fat and muscles(protein) are reduced together simultaneously. And if it is second case, then what should I have to do to burn(reduce) only body fat and not my muscles.
If you start reducing your daily caloric intake you can run the risk of your body breaking down the muscles to use them as a protein source. This will only happen if you reduce by too much. For example, if your daily caloric requirement is 2000 calories and you reduce that down to 1000 then you will certainly lose muscle along with fat. That is if you would be losing fat at all which I'm not sure you would be.
Based on the above my advice to you would be as follows: Use a calculator that can determine how many calories you need based on your age, height, weight and your weekly workouts (how many times a week you workout).
Once you have determined how many calories you need to maintain your current weight try and stick to that amount of calories. This will help you maintain your muscles, and even build muscle if you eat the right amount of protein, all while you burn fat.
http://www.freedieting.com/tools/calorie_calculator.htm This is a calculator that can determine your daily caloric needs. One thing I will say about this calculator is to take the number with a grain of salt.