"Firstly, if you've been inactive for over two years or have a medical condition, be sure you get a doctor's clearance to exercise.
The best whole-body exercise to start off with is walking. It is low-impact, you can make it convenient to where you live and you can make it pleasant by walking a scenic route, going with a friend or listening to music. Find sneakers that are comfortable for walking. Start off walking short distances: 10-20 minutes at a speed that makes you breathe harder, a few times per week. Slowly increase the time you are walking by a few minutes each week – too many people try to start with too much and exacerbate injuries or it becomes too much! Remember, every bit counts, so warming up and increasing it gradually means your muscles and joints won't become too sore.
Don't forget to use other opportunities in the week for exercise as well, eg take the stairs, park further away and walk 10 minutes to and from work.
Other low-impact options include swimming or aqua aerobics or cycling or cross trainers. Strength training can also be included in your workout with professional advice."