Have good coffee beans, stored properly. Try a roast that is made primarily for bialetti mokka pots. Grind them before use.

Boil over low flame. Let the coffee rise after turning off the flame. Too much heat will "burn" the coffee. If your coffee is still too bitter, try a coarser grind. Unlike in an espresso machine the grind should not be as finely ground, nor should you ever tamp it. Water needs to rise through the grind easily. Alternatively try filter coffee if it works in your setting. Enjoy improving.