Just enough to sweat a little, breathe a little faster, and increase our heart rate some. Either one or both. I highly recommend getting in touch with an exercise physiologist for a few sessions before buying any equipment or joining a health club. After all, if you wanted to take up golf, lessons from a pro would be absolutely necessary. This may be the best money you ever spend on your health.
Good habits are as hard to break as bad ones.
Learning the correct way to lift weights and to use exercise machines is a must. The strength training discussed here is for the average person, certainly not one training for sports competition or for the bodybuilder or power lifter. Go to a gym or health club that has someone specially trained in exercise physiology, not just a person who has been hired off the street who can demonstrate the equipment and amenities of the club and possibly pressure you into signing up for an expensive membership. Get a complete tour before you sign up. Talk to other members.