Join a class with others in your age group. There are seniors' mall walking clubs, water exercise classes, senior stretch programs, even chair aerobics classes. Check your local YMCA, YWCA, or county recreation program. Start your own program with a partner, such as a relative or friend. If you feel too awkward or embarrassed to exercise, join the club.
Most people feel slightly silly when they start out.
Exercise is not just for skinny minnies. In fact, once you get going, look around. Few regular exercisers have perfect physiques. Most fitness buffs will respect your efforts and root for your success. If you're trying to lose weight, even a modest amount of regular physical activity can help.