Since you're starting out spend the majority of your time getting your technique right. Work on your stance, how to properly throw your punches, and how to move around properly. Otherwise you'll be wasting your time. Once you've put enough effort in learning the motions you can start off with a a few rounds on the heavybag. For each round work on some particular things, for now you can spend some time working your jab, throwing some 1-2's, and learn how to properly move about the bag.