First game I played was with Castile, I highly recommend playing as them. You are large enough so no one will try to jump without reason. There are some nice events (which with a reasonable chance will fire) giving you personal unions over Aragon and possibly Naples, thus giving you insight in that mechanic of the game. Then you have some rather easy targets at close range, Granada and the Maghrebi nations. And then you just go colonizing which lets you expand and become rather powerful as well as giving you loads of easy targets over sea to conquer.
Gamewise it's definitely most like CK2 of the ones you stated, yet rather different. Characters have little to no meaning at all, apart from their monarch skills.
As
@grotaclas already stated, read every single tooltip as they come. Fighting a battle, read the tooltip. Trying to colonize a province, read the tooltip. Play around with the different mapmodes and read the new tooltips that appear there.
Quill18 has a great tutorial for complete beginners with Castile on youtube, although old so some of the mechanics have changed. But it gives good insight in most mechanics.