Hello Jeana! You can walk into a restaurant and ask if there is availability or any cancellations, but the chances of getting a table will not be good. I would not recommend this. If they ask you to wait, the time may also be quite lengthy.
My first step would be to contact Disney Dining at (407)939-3463. They will be able to assist you in finding availability at restaurants around the Walt Disney World Resort. All the information is at their fingertips. I have been going to the WDWR for many years and have always utilized a cast members help by phone.
My second step would be to approach the concierge desk upon check-in at your resort. They will be able to help you in locating availability as well. They are always very friendly!
So, it may turn out to be your lucky day and you do get seated as a walk-in...it does happen! My experience has taught me that this is not the best way. I have also seen too many people turned away by trying this. I would first try my two options above.
Have a magical trip and please let me know if I can help with anything else.