My weight is 104 KG, had three missed abortion, how to reduce my weight?

My gynecologist has clearly told me if I want to be a mother, I have reduce my weight, but I have no idea how. I have taken diet of a dietician, also went to a Gym, but not able to do anything their due to my excess weight. Any one there to suggest something. Anything which can help me 2 reduce my weight? All tests including Thyroid, Sugar level are normal.