>What nation should I pick?
Any nation you like, but focus on early age or middle age.
Avoid nations that utilize blood magic, underwater nations, cave nations, avoid popkills nation like ermor or asphodel.
>What pretender should I make?
An awake "monster" with earth magic, then give yourself the "hard skin" bless.
Noteable good expander monsters: Earth Serpent, Drakon, thrice horned boar, the point is to get a pretender with a high level "natural protection" (16-20 protection) and then add the earth bless on top of that, making them practically unkillable by regular humans.
Pick the monster and get the earth bless, pay for the bless and monster by going high heat, turmoil and possibly sloth if need be.
Keep in mind your "bless" will only be active for your pretender in friendly dominion, stick to attacking provinces with your religion basically.
>When can I join a multiplayer game?
The standard rule of thumb is owning 15-20 provinces by turn 12. If you can do that you've set yourself up for a decent start. So do a few test games versus the AI until you can pull it off reliably.
However, keep in mind that there is only one winner in every game, and so enter the game with an expectation to get your ass whooped the first match, it happens to basically everyone. You'll learn a lot though and that will be fun. Watch Youtube videos, ask in threads and use the Dom