If you're not already in shape, I suggest some moderate exercise (walking) to get your legs ready. Being your first trip, you will want to see so much that you won't realize how tired you're getting and it really helps if you do a little "training" first.
Comfortable footwear is essential - you'll find lots of suggestions on the boards. Most popular are
Crocs, Tevas, Birkenstocks and New Balance. Go for comfort not cuteness (Personally, I think my red crocs niles are really cute and love my Birki clogs)
Bag or no bag in the park? In the past I've used a backpack, this past trip I brought a medium sized zip top tote bag to hold my wallet, travel sized pack of wet wipes, water bottle, sunglasses, phone, camera, hair brush and lipstick (gotta look cute for pictures). This worked great and was not heavy to carry at all. They do check bags at the gate but they just open it and look inside, doesn't take very long.
If you don't bring a bag it helps to have pants/shorts with large pockets so you can bring your room ID (can also be your park ticket and you can use it to charge purchases) and camera of course.
Fastpass - get these as soon as you enter each park. Some attractions such as Test Track and Soarin in Epcot run out of fastpasses by early afternoon and these are can't miss attractions.