The general definition of interval training is; alternating burst of intense activity with intervals of lighter activity.
Besides checking with your doctor, please realize that intervals are intense , so you should only start with one or two sessions per week ( this allows 2 or 3 days for recovery and growth).
Start with 20 minutes, and after 6 to 8 weeks of training, increase your regimen to 40-60 minutes.
The wonderful thing about this form of training, is the quick results. It is not uncommon to see noticeable results in 2 to 3 weeks! This is twice as fast as traditional aerobics, and possibly the most satisfying fact is: your body continues to burn calories for 48 hours after you work out!
Unlike a lot of exercise regimens, Interval training works both the aerobic and anaerobic systems, ex: during the high intensity efforts, the anaerobic system uses the energy stored in the muscles (glycogen) for short bursts of activity, and works without oxygen, but uses lactic acid.
Now, during the recovery, the lungs and heart breakdown the lactic acid. This is the phase that the aerobic system is using oxygen to convert stored carbs into energy.
To get you started here are a few interval exercises:
Squat jumps,chest pecs, bridge,stair steps,plymetrics (jumping). Lastly, make sure you raise your heart rate, to the point that you work up a sweat. Now let's go!