Difficulty determines the BadBoy decrease rate (BB is a value you get when behaving 'badly', ie DoWing - Declaration of War - or conquering provinces, or diplo-annexing countries). It has a high impact on your starting treasury, and on various results in the game, tough not much on economy (including trade). No effect on the quality of the AI, except the AI cheats more the higher the difficulty.
To discover the game first, I advise you to choose Very Easy/Coward and an easy country (1419 China, 1492 Spain, France, and the like), to get an easy game.
Check my sig, and choose France in 1419.
