There’s really no etiquette to worry about, per se. If the server is public (non-password protected), it’s expected that you’re free to join and fly, obviously while avoiding being a d*ck to others, which I doubt would be an issue.
Most servers have CTAF freqs listed in the briefing; some are all combined and some have separate freqs like in real life. Basically see-and-avoid and act like you would at an uncontrolled field in real life and you’ll be golden.
I’d recommend the Hoggit training server. There’s plenty of airports to choose from, decent ping, and friendly people who are all there just to train and practice, all doing their own thing (not a mission, per se, just a series of ranges and targets people do their own thing on).
If you go to Hoggit’s Reddit page, wiki, or discord channel you can find their guide to connecting and operating in their servers, but no need to overthink; as a RL Pilot you’ll be miles ahead of many players if you fly a normal pattern and watch for other traffic.
I second this; eventually you’ll want an environment you can control and administer yourself, not mention you should have a much more solid connection, and with only one plane around the server performance would be better. But that’s another can of worms, installing the dedicated server on a third machine. Not hard though, I’ve used my laptop as a dedicated server and runs it fine; most calculations are made client-side so you don’t need a workhorse of a PC for that, IMO.
There’s two competing schools of though right now: the game has a new built-in voice chat that works well, and there’s a third party plugin called SRS which works behind the scenes to realistically simulate UHF/VHF radio comms. Hoggit and most servers use SRS for now, since it came first and offers a lot of options. Set up is not difficult, just takes some doing. There’s a number of well done guides to setting it up and using it. You’ll map a key or button to be your PTT switch.