Watcher mode: Non-ironman games, there is an unofficial way. Press the ' key (or your region's equivalent) to open the console command prompt, type in observe/then enter, and the game's playing itself. To play as someone afterwards, type charinfo, which'll bring up much more detailed info about characters you highlight over. Make a note of any landed person you can play as (depending on DLC, theocracies are unplayable without modding), pick someone, type in play <character id#>.
As for good places to start, I learned on eariler patches and barely play non-GOT/ATE FF games, so my advice might not be current; with that in mind, 1066 Poland was where I learned the basics:
Holy Roman Empire will more or less leave you be, although feel free to swear fealty to them and conquer from within.
Pagan neighbors to your north amd northeast, so proper holy war timing.
Starting dude (Boleslaw the Bold) is pretty freaking competent at warfare and reasonably young. (mid 20's IIRC)
Barring that, same time period but Ireland.
As for amount of time to get how to play? Started with no experience in the genre, ~30 hours to not suck.