Hi Weight gain happens either due to addition of extra calories or due to failure to spend calories consumed...these extra calories deposited in our body as fat..for weight reduction you have to consume less calories and spend more...avoid junk foods,carbohydrates like rice etc,avoid oily foods...add proteins and fibre rich foods to your diet...do regular exercise at least 2 hours a day...consult dietitian fallow the diet chart strictly...you can see the beneficial effects within a month..consult orthopedic surgeon for knee problem