Knees in pain

I was in the hospital for a year for an illness I had for five years. I gained 120 pounds. I am currently working out and have lost fifty pounds, but my knees really hurt the more I excercise. Can you please recommend any excercises I can do to help my knees. I do not have insurance so cannnot go to therapy. I thought perhaps swimming would help. I did not have knee problems before the weight gain. Thank you for your time.