What should I do if I want I to lose about 40 pounds in six months?

Eat less - Do more. Eat about 500 calories less every day. Get a bit more activity every day. If you don't lose weight in a week, then eat less and exercise more. Repeat weekly until you are losing 2 pounds per week. Make sure you stay well-hydrated - at least one litre (quart) of water every day above the other liquids you drink. Go for it!