First you need to make sure you are not diabetic. One of the signs of it can be rapid weight gain. If you know someone with a tester test yourself after fasting for eight hours and then 2 hours after eating. The fasting one should be 90 to 100, the after eating one should be around 130 to 150.
Fat is not your enemy, sugar and refined foods are way worse for you. This means no candy bars, pop and no more than a serving of popcorn. Stay away from white foods. Eat enough to keep you going so your body does not go into starvation mode. Those candy bars he drops on your desk, throw them back at him.
Walk continuously for an hour a day at least 4 days a week. Get some strength training in a couple times a week.
I have lost ten pounds in the last two months by following these guidelines. Hope you have continued success.
Please do not compromise your values for weight loss.