Start by doing small amounts of exercise several times each day.
Stand up from your chair 3 to 4 times in row before each meal or 1 or 2 times during each television commercial break.
Rise up on your toes 5 times after you brush your teeth.
Take a short walk in the house each time you leave the restroom.
Lift soup cans up to shoulder level 5 to 10 times before each meal.
Progression
Add a few more steps to your walks every few days.
Add a few repetitions to your strengthening exercises each week.