You'll get plenty of good advice on this thread, but I'll put my 2 cents in.
1)As you mentioned, you are not in the best health. However, your symptoms (outside of bad back and knee) seem to be a general lack of conditioning. What I would NOT DO is dive in and become a "weekend warrior", get hurt even worse, and give up. I think you need to do the foloowing steps:
1)Meet with your doctor and tell him/her you are committed to getting in shape. Ask them for a referral to a GOOD physical therapist for an assessment. Also ask if your doctor wants to do a stress test or anything before you embark on your journey.
2)You did not indicate how healthy your diet is. If you truly are committed to getting in better shape, it is VERY important WHAT you put in your body. A healthy diet will help with weight loss as much as exercise.
3)Set a REALISTIC goal for yourself, and write it down. Make it specific, and put it up in a place where you can see and read it several times a day. Some people I know write their goals down on a card, laminate it, and carry it around with them all day, as a reminder.
4)Start SLOWLY. You may need to heal your back through stretching and NO-impact exercise before you can try more of the impact exercises. Water aerobics, or walking on a treadmill will help that. A DAILY stretching regiment is almost a MUST if you have a bad back or knee.
5)Reward yourself when you hit a goal, like walking for an hour on a treadmill, or losing 5 pounds. just something little to keep yourself on the path......
6)You will be AMAZED at how much better you and your joints will feel if you lose some weight. It may really help your knee and your back problems......
7)Have a good support staff. One of those resources is this very forum. Plenty of folks on here have gone through what you have, so keep us updated. Also, if you can swing it, get a family member or friend to work out with you. That is a VERY strong motivator for BOTH.......
Best of wishes to you.