So as a complete beginner, for the most part, all I see in online public lobbies is Nordschleife and Drifting maps, neither of which are very beginner friendly especially when said beginner is trying to learn the car/h-shifter/racing lines at the same time trying not to spin out and not hit anyone.
My question is, what map and cars do I focus on if racing with others is the end goal?
Personal opinion: if you are a total beginner then choose a short track (so you can learn it quickly and focus on the driving instead) and a friendly car (so you don't get mad at it). For the tracks: Silverstone National, Brands Hatch Indy, Vallelunga Club.. Cars: the Maxda MX5 Cup is fast and friendly, also the Formula Abarth. When you feel at home with those, try more powerful cars on those tracks, then you can move to bigger tracks, starting with the "long" versions of those three.
You can get a reference for fast times on those combos by using this website (and app):
http://www.radiators-champ.com/RSRLiveTiming/
(don't get scared by the "alien" lap times, when racing most times you won't need to be that fast)
Another piece of advice: if I have to prepare for a multiplayer race, be it for a league or something else, I do 3 complete race weekends against the AI on those track/car combos before the actual mp race. Practice, Qualify, Race, using same qualy and race duration as the actual mp race, AI at 100%. Not all three races together, possibly spread across a week. First one to learn the ropes, second one to become competitive, third one for the last improvements and general consistency. It may sound like a lot of work but it's a fun way to prepare for a race instead of hotlapping all alone and it will bring you the necessary confidence come race day.
As advised by others, SRS (simracingsystem.com) is a good and free platform for multiplayer in AC.