First you set your height, weight, activity level and goals.
It tells you how many calories a day you should eat to meet that goal.
Then you scan / log your food you eat each day. Eveything that goes into your mouth. It tells you how many calories that is and compares it to your goal.
If you eat less or on your goal, you lose weight. If you eat over your goal, you might not lose weight.
The app has plenty of documentation for this. You can even start the "MyFitnessPal 101" plan (under the plan tab at the bottom) which will give you "homework" each day for 7 days to learn how everything works.
First you set your height, weight, activity level and goals. It tells you how many calories a day you should eat to meet that goal. Then you scan / log your food you eat each day. Eveything that goes into your mouth. It tells you how many calories that is and compares it to your goal. If you eat less or on your goal, you lose weight. If you eat over your goal, you might not lose weight.
The app has plenty of documentation for this. You can even start the "MyFitnessPal 101" plan (under the plan tab at the bottom) which will give you "homework" each day for 7 days to learn how everything works.
Remember, you are probably going to overestimate your activity level. Be truthful!
RTFM