Do not tamp the grounds and overfill the basket, it might block the flow of water and create a safety hazard.
For both taste and safety, use coarse grounds. A coffee grinder (manual or electric) is a must-have complement of a moka pot.
If it's still too bitter despite the coarse grind, try a light or medium roasts according to your taste
Some people (including James Hoffman) believe that starting with hot water will result in less bitter coffee. Not everyone agrees with this. For instance in Italy nobody uses hot water. There is also a risk of getting burnt when using hot water as the moka pot is not designed to be handled when the base is full of hot water. For safety reasons this is not a recommended option. As a compromise, you could use water that is slighly warm.
A light or medium roast combined with fresh coarse grounds is generally enough to not get any bitterness.A moka pot is a pressure vessel and as such is not a harmless device, always follow the instruction manual provided by the manufacturer.
Mokapots are not designed to make espressos by tamping the grounds or other hacks. If you want to make espressos, you should use an espresso machine instead.
Regularly check that the safety valve is functioning properly (with a pin). Do not fill the water more than recommended by the manufacturer it might damage the pressure valve. Change the pressure valve and the gaskets regularly. There are made-in-Italy pressure valves readily available online. Regularly descale the metallic filters.
Disclaimer: this is for information-only, follow the procedures from the manufacturer, like Bialetti's user manual