Hi Babitha, and welcome to planDisney! What a fun question this is! I love your idea of preparing for a Walt Disney World Resort vacation by watching some classic Disney movies. I honestly can't think of a better way to get in the Disney spirit! 
I have to admit, your question had me a bit like Alice following the White Rabbit. I went down quite a few paths to get some ideas for you. It turns out you can watch just about any of Disney's amazing offerings and find something in the theme parks to reflect it! But I have a few favorites I think will truly inspire you as you plan your magical vacation.
 - 
Snow White and the Seven Dwarfs: Walt Disney's very first feature-length animated film was groundbreaking in its day and is the source for the popular 
Seven Dwarfs Mine Train ride.
 - 
Peter Pan: Flying high above London on 
Peter Pan's Flight will feel truly magical after seeing this movie.
 - 
Pirates of the Caribbean: The Curse of the Black Pearl: Watch for Captain Jack Sparrow popping up among the classic scallywags of the 
Pirates of the Caribbean ride!
 - 
Star Wars: If you haven't seen any of the Star Wars movies, I suggest skipping ahead to Episode VII - 
The Force Awakens. The inhabitants of 
Star Wars Galaxy's Edge are featured in that movie and the two that follow.
 - 
Up: If you spot 
Kevin walking around Disney's Animal Kingdom Park, you'll know why this movie is a must-see!
Avatar: You're transported to a distant planet in 
Pandora - The World of Avatar, so seeing the movie will make your surroundings more familiar.
If you have 
Disney+, I can't recommend 
The Imagineering Story highly enough. Through several episodes, it explores the history of the Disney Parks, giving you a whole new appreciation for this magical place.
And I didn't even get to mention 
Toy Story, 
Cinderella, 
Beauty and the Beast, and so many more! If you do an Internet search like I did, you're sure to find tons of ideas for what to watch before your vacation. Just look out for the White Rabbit!
Thanks again for bringing us your question. I had a lot of fun answering it! Please come back and see us again real soon!
Keep Disney Dreaming,
Kathy K.