Getting healthy starts in the brain.  If you think you can do it, you can...believe it.  Start in small ways to exercise (you will feel better) in any way you find comfortable.  Walking is great and not hard on the knees in general but your pain should be discussed with you doctor.  Make what you already know to be the right food choices and keep a notebook listing everything you eat and when.  That will make you feel accountable as to what you have put in your mouth, it's  right there in black and white and you can't deny it.  Keep "free" foods like lettuce, celery, air popped corn, etc around so there is always something  to eat when you just have to have something.   Don't go hungry.  Be patient and know that every day you are improving your body.  Time will pass and you'll be able to see the results.  P.S. drink lots of water, it will help you feel full and help flush your system.  Good Luck