I know the feeling well! I flew for the first time a couple of years ago and needed so much advice when it came to flying and flying to WDW.
You can check in online starting 24 hours before your flight. With most airlines I’ve flown (including United) I recommend checking in as soon as you can. Then you should be prompted to print out your boarding pass(es) for your flight there, or see an icon with the option. I print out my boarding pass and download the app to get my boarding pass on my phone to have my bases covered. You can usually print it out at the airport but I flew out of a small airport once and they didn’t have the kiosk at my terminal so I just do it to be safe.
If you’re staying onsite, for your way home WDW will put your magical express departure info in an envelope on your door the day before you leave. If you fly out of specific airlines and have seat assignments you will also get your boarding pass in that envelope.
I have found the WDW ME website to be helpful for my WDW specific tquestions. (
https://disneyworld.disney.go.com/guest-services/magical-express/ )
I don’t know the link but there is also a FAQ MCO thread on the dis.