For Crusader Kings 2, the first thing to learn are the succession laws, which most people recommend doing by playing an Irish count (in 1066, after the viking invasions). Put your chancellor in neighbouring territories and you have a chance of gaining a claim in several years.
Push each claim, and try to get your eldest son to marry daughters of other Irish leaders, for allies and claims (most daughters pass on weak claims which can be pressed if the title holder is a child, a cripple or a woman).
You'll work your way up from count to duke to king of Ireland, and by that point you should have enough knowledge of the basic game to play the Byzantine Empire, France, England, or any other major power.
