Have you discussed your knee pain with your doctor? If not, it would be a good idea to do so, so you can find a plan to help you move forward. You also don't have to do everything all at once, but start out small, perhaps start working on a plan to eat at least 5 servings of fruits and vegetables each day and/or drinking at least 8 cups of water each day. Once you start doing something small like those things, you will start to feel great about reaching some of your goals and perhaps want to add on more goals for you to reach, including exercise. I'm not able to offer exercises for you to do with the knee pain since I am not a doctor and I don't know your health history, but talking with your doctor about that will allow you to find what exercises are better for you to do and easier on your knee. There are plenty of exercises though, that you can do from a seated position. You may want to check out the Limited Mobility Lifestyle Center for some tips: www.sparkpeople.com/resource/lifestyle_cen
I hope your knee pain goes away soon!