Wear good comfortable (tried and true) shoes or sandals for lots of walking. Your feet will probably swell - drink lots of water! We always take a good fanny pack and / or backpack with hats or visors, Xtra sunscreen (I take a partial bottle so it's less weight), our camera, some of those mini fans and rain ponchos if storms are forecast (no purse for me).
Take advantage of the early openings / late closings that are available to resort guests! You can accomplish a lot of rides quickly + snag a couple of fast passes if you're there when they open. Just for Example at Epcot - head for something busy like Test Track - while one or two of you wait in line the others grab fast passes for everyone to come back later. Once you ride it once, go to Mission Space - wait thru the line & ride that and then by the time you're done you can go back to Test Track with your fast passes (or vice versa). The fast passes work well - but you sorta gotta get the hang of working the system since you can only have one at once. We'd rather use them on something like Peter Pan that has a 90 min. wait and wait in line 90 minutes for Splash Mountain just 'cuz of the type of ride that it is.
Also, we prefer to eat a very light breakfast (like pastries) and then stop early for lunch (like by 11:30). We don't normally eat lunch until probably 1 or 1:30 on the weekends, but if you wait for 12 or 1:00 the lines will be awful and seating limited. If you stop early, you can have a more leisurely lunch at a good table (clean) & you won't have to hover over someone else with your tray while you're waiting for them to finish so you can snag it.
We also don't care a lot about sitting out in the hot sun waiting for the afternoon parades - we've noticed that a lot of the lines are less during the parades 'cuz a lot of people DO watch them - so it's a good time to catch stuff like Big Thunder Mountain Railroad etc. But that's our preference - we'd rather watch the evening parades when there's no sun.
Lastly, it's hard not to "go commando" when it's your first trip - but do try to take a break in the afternoons to go back to your hotel & catch some pool time! You'll need to get off your feet & relax some too! The refillable mugs they sell at the resorts are a good deal - last time we went they were like $10 but free refills on anything you wanted at the drink stations - so we'd get our coffee in the mornings...take them to the pools in the afternoons etc. If you decide to get those, get them on day #1 so you have them for the entire week.
(Insert deep breath here....OK!) Go and have fun!!