What is the best method of starting aerobic conditioning?

Heart rate up & up! All exercise can be cardio! even circuit weight training! but running, walking, biking, or any number of indoor activities will suffice. Get your heart rate to at least 70-80 percent of its max: 220-age in years! and keep it there for 30m 3x per week. I advise all my sports med pts to get a heart rate monitor. Initiall your hr will be high for any level of effort, but will lower as you improve.