Wow, congratulations! Do you know what you'll be doing at Disney World?
I'm not sure where you're flying from or on what airline, but several airlines have you check in 24 hours prior to your flight, so do that on Saturday. If you don't have to do that, confirm your reservations (online or phone) just to make sure everything is correct.
Since you're leaving Sunday morning, the good news is that traffic to the airport should be pretty light. I would recommend you get there 2 hours before your flight. Since its your first time, it wouldn't hurt to get there before that.
Since you'll be going thru security, be aware what the TSA regulations are for what you can carry thru the security line. Make sure you have no knives, scissors and all that. Also be aware that some liquids can't go thru the line and some things that you can bring have to be in a quart size ziploc bag. Look at, and get somewhat familiar with, what is listed at
www.tsa.gov.
Of course, make sure you bring ID with you...a photo drivers license should do the trick. Also make sure that your name on your drivers license matches the name on your air reservation.
You should also confirm how you're getting to Disney from the airport. If you're renting a car, confirm your reservation as well (and it wouldn't hurt to check again, it might be cheaper!).
Try not to get too nervous; flying can actually be fun!