Hello Jaclyn,
I can just imagine how excited you must be feeling about your fast approaching 1st ever trip to Walt Disney World Resort next month, I love it when my own counter to Disney gets down into the lower digits!
For your Disney days it's such a good idea to carry daily essentials in a small across the body bag or backpack which can either be worn on an attraction or safely stowed away in the little net/mesh pockets which are provided on some rides; if you are at all unsure of the safety aspects of stowing your personal possessions on a attraction I recommend seeking the advice of the nearest Cast Member.
Multi-use, key-operated and single-use, coin-operated
Locker Rentals are conveniently provided across Walt Disney World Resort; all current charges, deposits and so on can be found in my previous link.
If I can help sprinkle any extra pixie dust onto your upcoming September trip to Walt Disney World Resort please pop back to ask me another question.
Have a great Disney day.
~ Jane