Exercising and eating maybe 4 small, healthy meals a day will help you lose weight. But if you want to make the most dramatic change, don't forget to include exercise. A diet with the best results includes exercise.
It is not a good idea to only eat one meal a day. Not only will your body go into starvation mode, since it will attempt to maintain energy rather than lose it, if it thinks there is no food coming in, but your blood sugar within your body will plummet and then after your one meal per day you will find that it soars, then crashes again.
This will give you feelings of hunger and tiredness/lethargy and a general feeling of not being 'on top'.
Moreover, the fact that your body has gone into starvation mode will make your body lose weight less quickly than if you were eating three or four small meals per day.
You may find initially that you lose weight quick;y, but after a few days this will stop when the starvation mode kicks in, so it is a really bad idea and you should instead lose weight by small, regular meals.