Food Diary

One of the best weight-loss strategies. The *food diary* is a record of your everyday meals.
All you need is a pen and a notebook (or your pc/cell/iPad) to make notes of the foods' calorie scores (kcal) and the exact level of your basal metabolism rate which means the amount of energy expended daily at rest. This is what your body needs to keep up the basic functions every day. To calculate it, here's the formula:

Basic Metabolic Rate - BMR - Food Diary

Don't worry it's the easiest and the best way to calculate how much calories your body needs to keep up the
basic metabolism and life-functions.

Here's the trick:
1 pound or 0.5kg of bodyfat equals to approx 3500kcals. If you want to lose 1 pound or 0.5kgs a week, you shall decrease your calorie intake by 500kcals/day. This is when you will need your notes of how many calories you consume a day. Then all you have to do is from the next day, you will just leave out some snacks and/or substitute them with fruits or other low-fat meals. :)

If you wish to gain weight you shall increase your calorie intake by 500kcals/day.

Please note, that the minimum calorie intake per day is 1200 for women, and 1800 for men. 
UK Department of Health Estimated Average Requirements (EAR) are a daily calorie intake of 1940 calories per day for women and 2550 for men.

How can you find out food's calorie levels?
In 99% of the cases, the product's energy-levels are written on the wrapper. So you just have to make a note of it in your diary. You can also find calorie charts via a search engine.
Another thing you shold consider to write in your diary, is the amount of calories you can 'burn' during a certain workout.  

Tip: To boost your weight-loss, you may have to look for the fat rate of the products too, record it as well and make sure that the daily fat intake will not exceed 30g. This way you can lose another 1 pound or 0.5kg a week. 

Please note that the more hard your job is (physically) or the more workout you do a day, the more calories your body will use and 'burn'.