If you haven't read Hydroguy's tips, check that out first:
http://www.disboards.com/showthread.php?t=1520483 That will give you good ideas about what to expect as far as crowds, what rides get busy and which usually don't, etc.
Since it will already be crowded by the time you get there, I'd grab a FastPass for your favorite ride first. Then go hit some of the things that don't usually have long lines. If you are there for more than 3 days, it might be good to take your first day kind of leisurely, explore and enjoy, and keep collecting FPs as they become available, to use later on.
Some rides that often have short lines (or at least fast-moving lines) are: Winnie the Pooh, Pirates, It's a Small World, Pinocchio, Snow White, Buzz Lightyear (sometimes), Honey I Shrunk the Audience.... that I can think of.
Also it's a good time to take a ride all around the park on the train. Always a great introduction to your first day, to put you in the right spirit!
Have a great time!