from personal experience:
\- TF2Center first to get familiar with the main two formats and discover the class you enjoy the most, then look for newbie-friendly pug discord servers to learn strats and comms
\- To find a team, play scrims and official matches you need to register in a league from your region which would be ETF2L.org in your case and create a recruitment post for the Open division, your desired format and class you wish to play
\- Highlander requires more players, coordination and organization, 6s is significantly more popular though seasons don't overlap so you could play both if you wanted to
\- All leagues have their own division systems to determine player's skill. In ETF2L, from lowest to highest: Open-Low-Mid-High-Premiership
\- it exists
\- Varies from league to league, but from what i've seen ETF2L can be either very lenient or totally petty when it comes to stuff like toxicity, homophobia and racism
Tips:
Don't be afraid to ask for help, i've met some of the nicest and smartest players in this community because i reached out when i didn't know something.
Don't be ashamed of being new and playing poorly, everyone starts somewhere
If you have any questions you can dm me here anytime