Did your doctor give the all clear for doing exercise ? Ideally, a person should always check with their doctor before starting an exercise program.
However, if your doctor has given you the all clear for exercise, then you might try a daily walk. Can you walk without any pain or discomfort to your knees ? If so, start with a daily walk. Doctors will tell you that a 30 minute brisk walk each day can reduce your risk for heart disease by over 30%.
If you find it difficult to walk, do you have access to a pool ? Swimming or water aerobics could be something you could do. being in the water takes stress off a person's joints.
Do you have a gym membership ? If so, try the recumbent cycle. That's the stationary bike you peddle in a reclined position. You may find that less stress on the knees too.
If you can't stand for long periods, then sit in a chair and do upper body strength exercises with resistance bands or hand weights. Work your upper body muscles if you can't work your lower body ones.