There is an in-game tutorial, which isn't bad for showing the basic mechanics. I'd start there. There are a few tutorials on Youtube (and probably other places) but it all depends on how old they are and which expansions you have, because things have changed a bit over the years that CK2 has been around.
After that, jump into the game and just... play around. Make frequent saves, take chances and if they don't work out, go back to your last save and try something different to see if that makes a difference. Be prepared to do this a lot - or, just accept the mistakes and go on.
Don't try ironman until you feel like you've got more of a grasp on things. CK2 is a complex game, it's going to take a long time to get good at it. And don't compare yourself to the experienced players here on the forums here and feel bad that you can't do what they can. Most of them have thousands of play hours invested in the game. (I have about 1000 hours, although some of that is from leaving the game paused while I do other stuff, and I still consider myself only semi-experienced.)