Can someone please tell me how this site works?

At it's core, this is a calorie-counting website. You plug your stats (height, weight, etc) into the settings area and choose a weight loss goal (rate of loss, e.g. .5 pounds per week). The system will calculate your calorie goal and give you a basic maco (fats, protein, carbs) split. You then use the food database to track your intake and the exercise database to log your workouts.

The MFP model does not take exercise into account when coming up with your calorie goal, which means if you set it as a .5 pound per week rate of loss, you should lose at that rate just by hitting your calorie goal without any exercise. If you do engage in physical activity for exercise, eat back some of those calories, but be careful if you're using the calorie burns provided by MFP. They are notoriously inflated and can erase your calorie deficit if you eat them all back.

Check out some of these posts for more help and good luck!

http://community.myfitnesspal.com/en/discussion/1235566/so-youre-new-here

http://community.myfitnesspal.com/en/discussion/1399829/step-by-step-guide-to-losing-weight-with-myfitnesspal

http://community.myfitnesspal.com/en/discussion/1234699/logging-accurately-step-by-step-guide/p1

http://community.myfitnesspal.com/en/discussion/1296011/calorie-counting-101/p1