The cornerstone of any good fitness plan is getting your diet right, and to do that, the first thing you need is, to determine what your daily Caloric Needs are. A crude way to find out your daily Caloric Needs is to multiply your target weight (not your present weight) in pounds with 12. As an example, my target weight is 67 kgs which is 148 pounds. That makes my daily calorie intake 148 X 12 = 1776 calories.
If you wish to look like a fitness model, then you have to fine tune it. But if your goal is to get in really good shape, lose weight and look lean and fit, then managing your calories this way works well.
Now that you know your Caloric Needs, it will be good to know your macro nutrient breakup. This is a guideline. First determine what your body type is
Ectomorph: Naturally thin. Skinny limbs. Struggles to build muscle/gain weight.
Mesomorph: Naturally muscular. Wide shoulders. Fits right between ectomorph and endomorph build.
Endomorph: Naturally broad. Wide hips. Shorter limbs. Gains weight easily.
Now convert these percentages into calories. I am an Ectomorph. So, in my case, Protein = 444 calories, Carb = 977 calories, Fat = 355 calories. Total 1776 calories as determined earlier.
Converting those calories to grams: Protein and Carbohydrates contain 4 calories/gram. Fat contains 9 calories/gram. So, my protein requirement becomes Protein = 444/4 = 111 gms, Carbohydrate = 977/4 = 244 calories, Fat = 355/9 = 40 gms.
So, my baseline macro nutrient requirement is as follows:
Protein = 111 grams, Carbohydrate = 244 grams, Fat = 40 grams.
I will check my measurements on the scale after a month and review again and adjust according to my goals.