I hope you took the time to write down your goals. Now lets move on to arguably the most important factor to consider when aiming for overall fitness: nutrition. Although I myself would generally use the term "diet", nutrition is a better descriptor as "diet" inherently indicates something which you do for a short period of time. Good nutrition on the other hand ought to be a life long habit.
The first step in designing a good diet is to know how much your body really needs. Yes, I am talking about calorie counting. If you think that's too extreme then let me tell you that a majority of people fail to get optimal results because they have no idea of how many calories they are actually consuming. It's a habit that will take a little time to develop but it will help you for life. You can find online calorie counter for most food items (like thecaloriecounter.com), also packaged product have their nutritional information printed for anyone who cares to read.
In order to calculate your daily calorific need use the Harris-Benedict formula to find your Basal Metabolism Rate (BMR) first:
BMR (Men) = 66 + (13.7 X wt in kg) + (5 X ht in cm) - (6.8 X age in years)
BMR (Women) = 655 + (9.6 X wt in kg) + (1.8 X ht in cm) - (4.7 X age in years)
So, for a 24 years old guy who is 176 cm. tall and weighs 62 kg. the BMR would be 1632.
Now, multiply your BMR by your activity factor to find out your daily calorific needs. Activity factor is given for different cases as:
Sedentary = 1.2 (little or no exercise, desk job)
Lightly active = 1.375 (light exercise/sports 1-3 days/wk)
Moderately active = 1.55 (moderate exercise/sports 3-5 days/wk)
Highly active = 1.725 (hard exercise/sports 6-7 days/wk)
Extremely Active = 1.9 (hard daily exercise/sports & physical job or 2 X day training, marathon, football camp,contest, etc.)
Such that if the guy in the example is visiting the gym 5 days a week and working out for an hour (A.F. = 1.55) he needs 2530 calories to maintain his current weight. If he wants to cut down, a deficit of 15-20% is suggested (a heuristic figure is 500 calories deficit). He would be ill advised to cut down calories too much as in such a case starvation response would kick in and his metabolism would plummet. For bulking he should go in a similar calorific surplus.
Now that you know how many calories you need, it's time to decide on your macronutrient ratio (Protein, Carbohydrates and Fat). I started with 30-50-20 which is a good starting point for most people. For a 2200 calories diet this translates to 660, 1100 and 440 calories for protein, carbs and fat respectively. A set of handy relations for gm. to calorie conversion is:
1 gm. protein = 4 calories
1 gm. carb = 4 calories
1 gm. fat = 9 calories
(fun fact: 1 gm/ml of alcohol = 7 calories, huh!)
You can now convert your calorific requirements to requirement in gm.
Heading towards the conclusion, let me present the algorithm for diet design:
Step 1: Choose a lean protein for every meal.
Step 2: Choose a starchy carbohydrate for every meal.
Step 3: Choose your simple carbohydrates for your breakfasts.
Step 4: Choose your fibrous carbs for your lunches and dinners.
Step 5: Add essential fats if insufficient quantities are present in your foods.
Step 6: Count your meal subtotals and grand totals.
Step 7: Compare your totals to your calorie target and adjust the serving sizes.
Step 8: Assign a time for each meal.
Here lean proteins include different meats, seafood and low fat dairy products. Starchy carbohydrates include oats, potatoes, carrots, daals, beans, brown rice and whole grain products. Simple carbs are your fruits and 100% fruit products. Fibrous carbs include green veggies, tomatoes, mushrooms, etc. Essential (mono and poly unsaturated fatty acids) include nuts, virgin olive oil, natural peanut butte and fish fat.
Time for an example I guess. Here's what I have been eating for close to 5 months now:
Required Intake = 2200 calories
Breakup = 500, 500, 400, 400, 400
Food source (serving size) - calories protein(gm.) carbs(gm.) fat(gm.)
1. Milk (500 ml.) - 185.0 17.0 24.5 2.5
Oats (35 gm.) - 136.0 5.3 22.0 3.0
Banana (1 = 100 gm.) - 105.0 1.2 26.7 0.6
Almonds (9 = 10.8 gm.) - 63.0 2.7 1.8 5.4
Sweetener
Total - 489.0 26.2 75.0 11.5 (approx. 20-60-20)
2. Chicken Breast (100 gm.) - 176.0 31.6 0.0 4.6
Lentils (1 cup ckd.) - 230.0 18.0 40.0 0.0
Cucumber (1 = 300 gm.) - 45.0 2.0 10.9 0.3
Total - 451.0 51.6 50.9 4.9 (approx. 45-45-10)
3. Whey (1 scoop) - 130.0 24.0 4.0 1.5
W. W. Bread (2 slices) - 120.0 3.8 24.0 1.0
Peanut butter (1 tbsp.) - 100.0 3.5 3.5 8.0
Tomato (3 = 186 gm.) - 39.0 1.5 8.7 0.6
Total - 389.0 32.8 40.2 11.1 (approx. 34-41-25)
4. Eggs (6 whites, 1 whole) - 177.0 27.3 2.4 5.0
Potato (1000/7 gm.) - 124.0 2.4 28.6 0.1
Carrots (1000/7 gm.) - 58.0 1.3 13.7 0.3
Cauliflower (150 gm.) - 37.5 3.0 8.0 0.2
Total - 396.0 34.0 52.7 5.6 (approx. 35-53-12)
5. Shrimps (84 gm.) - 90.0 17.2 0.8 1.5
Lentils (1 cup ckd.) - 230.0 18.0 40.0 0.0
Cabbage (2 cup) - 56.0 4.0 11.6 0.8
Peanuts (10 gm.) - 34.0 1.4 1.3 2.9
Total - 410.0 40.6 53.7 5.2 (approx. 40-53-12)
GRAND TOTAL - 2135.0 185.2 272.5 38.3 (approx. 34-50-16)
As you can see, I also employed calorie tapering (reducing calories later in the day) which is a strong technique for weight (fat) loss. Sweetener is left blank coz when making this I was unsure whether I should go for artificial sweetener. The problem is that most sweetener contain either aspartame or sucralose as sweetening agent and both are a bit controversial as to their effect on health (I read somewhere that a possible side effect is death!). So, Indian friends lets just stick with sugar, American friends can try agave nectar or date sugar.
Last but not the least, I understand that this diet looks bland and tasteless but hey you can try with different spices. Add chopped onions, lemon or some herbs to improve the taste. Once or twice a week go ahead and satisfy your urges, eat whatever you want (it's your cheat day!). Reward yourself with a little pizza or brownie if you achieved your weekly but do not over-indulge! As you progress you might have to cut from two to one cheat meal but that shouldn't be an excuse for not starting altogether!
That's about it without getting into vague details and metabolic mechanisms (most of which I don't remember myself). Hope this will help you develop a habit of eating healthy and making better choices when it comes to diet.