Hi, I turned 20 this year and as a 20 year old male I am 5'7 and weigh 85 kg. I understand that its overweight and I'm at the verge of being Obese. I really want to cut down some weight and get healthy but the problem is I have a horrible knee problem, I have arthritis and I have also dislocated my knee, thus whenever I run or walk for a short distance my leg starts aching.
Only "heavy part of my body is the belly and the chest. And due to my over weight I suffer from acanthosis nigrican (I am not sure if being over weight is the reason but im fairly certain it is).

So Im in University, I got a gym for free, Im not busy after 7 so how can I work out, what kind of workout and for how long, how do I change my diet?

Thank you very much