I would say that a good start to learn the game fulfils the following:
- Reasonably small. You don't want to start as an emperor or a king of a big kingdom as it is very likely that you'd get a rebellion before you knew that you had done anything wrong.
- Reasonably secure. Don't start as a count or small duke next to a powerful realm of a different faith, and powerful rulers of your own faith aren't always safer neighbours, particularly not if they have de jure claims.
- Reasonably friendly for early expansion by war or marriage. You don't want to be the weakest realm/vassal in the area.
- Non-heretic feudal Christian in Europe. The Christian mechanics are somewhat easier than those for other religions (and don't require a DLC), feudals don't have to get out of tribalism or have to manage patricians and trade posts, and starting outside Europe as a Christian likely means Abyssinia or Georgia, both of which have some problematic neighbours that easily can cause game over.
Some ideas with their advantages and disadvantages:
- Any relatively strong vassal duke in th HRE. You are completely safe from foreign realms (unless you become the emperor) and can try to expand inside the realm through war and marriage. Disadvantages include that you could become the emperor (which isn't recommended unless you know what you are doing), that the emperor might enact laws to restrict vassal warfare, that the emperor could decide that he doesn't like you and imprison you or revoke your stuff, and that your vassals potentially might be elected emperor, which would make them your liege rather than your subject.
- The duke of Bohemia or Duchess Matilda of Tuscany (both in the Stamford Bridge bookmark). Somewhat similar to the above, but both can relatively easily form kingdoms before their heir takes over. Matilda has some expansion opportunities in the Italian Peninsula (and sometimes the Balkans and Africa, depending on what happens there) and the duke/soon-to-be king of Bohemia can try taking land from Poland or Hungary when they are weak and eventually lead the charge against the pagans in the Baltic/Russia. As both are foreigners (and Matilda is a woman, which disqualifies her) it is less likely that you'd get elected emperor, but if that happens you'd probably face larger problems from rebellious vassals as the Germans won't like a foreign liege, so keep that in mind. Matilda also must marry matrilineary unless you want the game to end.
- Duke Robert of Apulia (Stamford Brdige). You start fairly strong in the southern part of the Italian Peninsula (de jure in the kingdom of Sicily) and should be able to holy war your way to become king of Sicily before you die if you are careful (make sure to allow your levies to replenish between wars and try to get some money for mercenaries) and can then expand into Africa. Disadvantages include nearby republics getting greedy, the Fatimids or other Muslim powers in Africa potentially opposing you, the HRE possibly getting claims on you or heading into Africa before you can do that, the Byzantines deciding to press their de jure claims on anything in the kingdom of Sicily, and that quite a few of your kinsmen are landed vassals (and potentially claimants) or might decide to go adventuring (if you have tOG), which could cause some trouble.
- The kings of Castile, Leon, and Galicia (Stamford Bridge). They start relatively strong but with claims on each other. You will very likely end up fighting or killing your brothers to keep yourself safe (or possibly just because you want their stuff). After that you can either expand via marriages in France (the duke of Aquitaine has some nice daughters, and if you arrange the right accidents you might inherit all of his stuff, even if that has some risks), via claims or marriages in Brittany or Ireland, or, more historically, via holy wars in the Iberian Peninsula. Dangers come from the Muslims to your south potentially uniting against you, France or the HRE possibly wanting to get into the Peninsula, overly aggressive merchant republics wanting to get into the Peninsula, and the fact that you very likely will end up with three kingdoms and gavelkind succession early on, which means your realm will split apart on death unless you only have one son (or one daughter, if all sons are dead/disqualified) or form an empire (you could go with elective, but that means that your vassals migth decide that they don't like your chosen successor and that they want to vote for someone else (possibly themselves) instead.
- More or less anyone in Ireland (Stamford Bridge). Barring Scotland (rather unlikely) or England (proably less likely than Scotland) getting involved early on you should be able to fabricate claims and use mercenaries to become the king of Ireland, after which you can slowly expand in the rest of the British Isles. Advantages are that you are very safe (everyone in Ireland starts at close to the same strength and foreign intervention is relatively rare) and that you get a lot of time to learn the mechanics as you successively get more vassals to worry about. Disadvantages are that you will be playing as the underdog for a long time, that any foreign intervention is likely to be a big problem, that you are relatively reliant on claims and mercs (which is costly and slow) to expand, and that you likely will miss out on a lot of the early holy warring (which can be fun and is an excellent way to get land without claims) except for the crusades themselves (I would advise that you try not to win Jerusalem early on unless you know what you are doing as the nearby Muslims will want to take that back and are rather powerful). As an Irish lord, you can also get out of gavelkind very early on if you want to use tanistry (basically elective, but restricted to your dynasty), but keep in mind that your vassals might like distant cousins more than your chosen heir and that they themselves might pick up tanistry to try to foul up any plans you have to inherit their stuff or to merge their subrealms together to potentialyl threaten your rule.