Crusader Kings II, where to start?

Be glorious Mongol Horde and take over world

I started my first games in Ireland, just because at the starting time it was divided into many parts iirc. Makes it a little easier to start building your empire.
Also don't be afraid to mess your first game up, CK II is pretty complex and requires some playtime to get into, but once you get the hang of it, you'll love it for that.
Finally I suggest you look for some let's play videos on youtube if there are things unclear or if you want to know something more before starting a game. Some youtubers actually explain most of the game pretty well.

Start in Ireland, it's known as the Tutorial Island in Paradox grand strategy games.

look for quill18 on youtube...he have alot of ck2 on his site...his even sponsord by paradox nowday becous of how much he plays the games

Maybe the CK2 Wiki Beginner Guide would help?
http://www.ckiiwiki.com/Beginner%27s_guide

Ireland is easy for beginners but I prefer to play in Spain (Duke of Porto or Count of Santiago, depending on the bookmark) and reconquer the peninsula before taking over the world. Love this game.

Some hints for things that are not very intuitive:

• when marrying your heir, choose good traits (genius, etc) instead of titles or claims... it will help you in the long run
• Always try to matrilinearly marry your female descendants
• Use appoint as successor of Bishop if you wish remove a descendant from succession titles
• Give titles to people in your dynastic line, they like you more (but don't make them too powerful, in particular if they are claimants to your titles)
• Give Duke titles (you can only have 2 without penalties) to some kinsman with low power (single county, for instance). It doesn't matter if they don't have any territory in that duchy (their problem, not yours).
• Revoke titles whenever you don't incur in penalties.
• Invite people with claims on territories you are interested, marry them to your line/give them a title and then press their claim to vassalize those territories.

Have fun :)

You should aim for an independent Duke (yes, one of the duchies in Ireland should be a good start).

play someone in France or England is a easy choice, be prepare for restart a new game, And add find anyone in your friend list do play it a lot, ask him/her directly what did you need to know .

Ireland is a good starter. You start as a count which is the simplest to govern and slowly build up into a duchy->kingdom->Empire basically learning all of the steps and different playstyles as you advance.

Since the place is isolated you won't have any problems, unless of course you have the aztec DLC (which makes for a fun late game) or of course start with the viking DLC.

After that I suggest you try a Byzantium game and reform/reunify Rome.