Stay on property! I’ve been to Disney 3 times. The first, we didn’t stay on property, but we did the last two times and I have to say the experience was just so much better.
If you stay on property, you get to go to the Extra Magic Hours that different parks have on different days. Most people don’t stay that late, so you get to see some really cool stuff. We went to Hollywood Studios one time, and were hanging out near the back of the park where all the shows are, and it was EMPTY.
You also can take the shuttles if you stay on property, which makes things so much easier.
If you like shopping, go to Downtown Disney at LEAST once.
Make dinner reservations in July or August, especially for the popular restaurants like the new Beast’s Castle. They really do book up that early. I’ve never used the meal package, but FH’s family did when they went and he said it was great.
Stay in a mid range or value-level resort. I’ve always stayed at the value ones and there’s nothing wrong with them, but the mid range ones are going to be nicer, without a price tag like the Grand Floridian has. You aren’t really there except to sleep, so there’s no point in going all-out. Especially for your first trip.
Make a point to see the Fantasmic show. It’s my favorite.
And get parkhopper passes! FH and I would go to one park in the morning, go back to the hotel for lunch and a nap, and then to another park in the afternoon. Usually the second park would be the one that had extra hours or a cool show that night.
Oh! And download the apps! My parents got one that would show how long wait times were and how many people were in the park. It was incredibly helpful when trying to pick a park for the day. You’d think they’d all be relatively even, but it was surprising!