Wait until you are in your early teens to begin the baritone saxophone. Start on the other saxophone family members first, as these don't require as much air and are less hefty to lift. You must have the physical capability to achieve a good sound, and that will come with mature lungs and muscles.
Take more of the mouthpiece into your mouth than with the other saxophones. Place your top teeth where the mouthpiece reed meets the rails.
Loosen up your lips. A good tone on the baritone requires looser lips than with the tenor or alto saxophones.
Lower your jaw. The baritone sax requires a more open mouth cavity than the alto saxophone.
Increase the amount of air you blow. You will achieve a stronger, heavier sound than if you use the same amount of air as other saxophones require.