Not sure of your girl's ages, but I'm going with pre-school-elementary age which I know about!!
Number one - make a priority seating reservation for a princess meal - check out restaurant boards. Not sure how much time you have, but if it's over 90 days, go for the castle after reading up on the boards. My DD's loved it SO MUCH!
Go EARLY before parks open and be ready for fun! If your girls are in the age range I mentioned, head for Fantasyland in MK and do it all - follow advice for touring plans on the boards or in the Unofficial Guide to WDW - my family thanked me so much!! After that, have your day planned loosely, knowing what you want to see the most.
Get the WDW for Kids by Kids book and let the girls look through. They can tell you what's important to them and IMO that's what will be important to you!!
I mentioned princess meals, but any character meals are GREAT - you should probably schedule one or two. We did Crystal Palace with Pooh and CRT - planning to do Chef Mickey's next time! That said, how about Chef Mickey's on arrival night? That would start the magic early!!
My girls were 7 and 9 last year on their first trip - I would be glad to answer any specific questions you may have if it would help! It was so magical and everything I had imagined...their little mouths really did hang open as that train rolled in for the opening ceremony at MK - they were hooked!!!! Have a WONDERFUL time planning and enjoy your trip!!