Some of the most common reasons for knee pain are sprained ligaments, meniscus (cartilage) tears, tendonitis, and runner's knee...some other causes include bursitis,osteoarthritis, patellofemoral pain syndrome etc...Consult nearest orthopedic surgeon they will do the needful...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 with green tea,green leafy vegetables, fruits...do regular exercise at least 2 hours a day...good luck.