The goal is 30 minutes of moderate endurance activity on most or all days of the week. Every day is best but anything is better than nothing. Try to do all four types of exercise. Endurance, balance, flexibility, and strength. Try to do strength for all major muscle groups 2 or more days a week for 30 minutes. Avoid working on a the same muscle group to avoid muscle strain.
How hard should I exercise? Match your activity to your own needs and abilities. Start where you are and build up gradually. Listen to your body as you become more fit and increase your exercise as you tolerate, gradually make your activity more difficult. The more vigorous the activity and the more time you spend doing it the more health benefit you will see.
Remember to always talk with your doctor before starting any exercise program to make sure you are physically able to do it.