The finer you grind, the more resistance you create, the higher pressure you need to extract the coffee, the higher the water temperature you need to create the pressure.
Try to go as coarse as you can get away with whilst still creating some viscosity in the brew. This will avoid bitterness.
If you have a grinder, I always suggest giving the Sainsburys Tanzanian Mochi peaberry a go. If you don't like it you haven't really lost anything and you have had some practise at different grind levels.
Boil the water in a kettle first to about 80-90%. Then into the mocha pot and put it on a very low heat on the stove with the lid open. You want the coffee to ooze out of the tower very slowly. Take of the heat before the crema gets light and frothy.
I've acquired a Moka pot for Xmas - any tips on the best coffee to use/handy hints/things to avoid? Thanks