Losing 40 pounds by Sep 14 is a pretty tough goal; that's only 12 weeks away. Safe weight loss is 1-2 pounds/week; you'd have to plan on losing a little over 3 pounds/week and wouldn't be able to backslide a bit. I think your goal is a bit optimistic.
You should first check to make sure you don't have an medical issues that might prevent you from losing weight, such as hypothyroidism or insulin resistance. Your doctor can order some simple blood tests.
You would need to calculate the number of calories you need/day, just to function (that means, keep your heart beating, kidneys, lungs, and other organs functioning), then drop off about 500 calories/day (or 250 and burn an extra 250). That would give you a weight loss of 1 pound/week. To lose more, you'd have to cut more calories or exercise more, which may not be safe.
Concentrate on eating more fresh veggies and lean protein, low/no fat dairy, etc. Limit fruit juices and opt for whole fruit (1-2 servings/day) because whole fruit has the added benefit of fiber. Opt for high fiber fruits, such as apples, pears, etc.
Make sure you get plenty of exercise. 30 minutes, 3 times/week should be the minimum. You can start slow and work up, if you aren't used to exercising.
Trying to lose 40 lbs by Sept is not realistic. It's already June 21st. Losing
weight too quickly is dangerous and not healthy. I would suggest making
an appointment with your family doctor, discuss the fact that you want to
lose some weight and let your doctor give your some suggestions for
a healthy food plan. Walking is good exercise, but I would recommend
starting to walk for about 20 to 30 minutes initially and increasing the
amount of time to give your muscles a chance to get used to the new
walking routine. I wish you well. Eve
Start off working out slowly and cutting back on meal portion sizes. Drink lots of water 8 glasses per day. Start with like 10 mins. of a workout and move up. 10 mins. goes by fast.