The First thing to start with is “finding out your maintenance calories” (the calorie at which you are presently maintaining your weight). You can find it out simply by tracking the food you are consuming every day and calculating its calories at least for a week and take an average of it. It will give you the value. Now once you are certain with the number of calories for your maintenance, you can decide the intake as per the goal.

For weight or muscle gain, you need to eat above maintenance calories.To lose fat, you have to create deficit. The amount of deficit also depends on your age, weight, gender, body composition, underlying morbidities etc.

It is safe to start with 20-30% deficit from your maintenance calories and see the results. After evaluating how your body is responding, it can be adjusted accordingly. Drastically reducing the calories will lead to slowing down of your metabolic rate and your progress may stall. The importance of weight training in gym along with adequate proteins in diet comes as a rescue when you are in a calorie deficit. It will help you preserve your precious muscle mass and hence your metabolic rate to some extent.

Optimum intake of protein is very important in a calorie deficit diet, so that you would lose minimal muscles and maximum fat. For gaining muscles, you need to eat above your maintenance calories, but caution should be taken not to go on calorie surplus suddenly. Just 10-20% more to it and adjusting it further, as per the results, is a good idea. Many other factors like, body phenotype ( Ectomorph, mesomorph , endomorph ) which is not in the scope of discussion here, would also decide about deciding calorie and macronutrients intake .