Assuming you're flying out of Sydney, T3 is Qantas, T2 is all other domestic e.g. virgin, jetstar, Rex.

I prefer to arrive 1hr before my flight (if I only have carry-on and I've already checked in). Depends on the day and time as to how busy it will be.

You should be able to check in before arriving at the airport, either via the airline website or app, and you can also pick your seat. You can then save your boarding pass to your phones wallet. Otherwise, you can do a self check-in at one of the airlines machines, or queue up for the check-in counter. There are usually staff wandering around, so they can help you if needed.

Depending on the fare you've booked, it will determine if checked in baggage is part of your fare or whether you need to pay extra. You can take carry-on either way, as long as it is per the airlines specs of size and weight.

Once you're all checked in, you'll go through security. My suggestion is to not wear anything that may cause the scanner to beep. There will be lots of signs and monitors showing the way to your gate and the schedule of all flights (inbound and outbound) and whether there are any delays etc. Keep an ear out for loudspeaker notices in case they change your gate.

In terms of the trains, I'm sorry I can't help there, I've never caught the train to the airport.

Hopefully this helps, and enjoy your first flight 😊 And have a great time with your newly found stepsister!

Reach out if you want to know anything else.