Flat foot and Knee Pain. Feel right.

Hi, I have flatfoot problem, i ankle gets twsited and i fall very oftn very often and i have constant knee pain in left leg between the knee joint in front side, I am also very obese i am 5'8 and i weigh 124 kgs, I am ready to make lifestyle changes and i wish to join a gym how ever last time i tried my knee used to hurt very badly. Need your suggestion what i should do as i have so much fat stored in my body i will have to do heavy workout to reach my 1st milestone of weight loss of 5 kgs in 6 months. How ever my knee issue is not helping me and i feel i have hit a dead end no see no where else to go . My mom is a diabetic and she get podiatrist consultation, i brought a insole based on his recommendation how ever it still doesn't feel right.