Hi Candice,
You must be so excited about your first visit to what we like to call the most magical place on earth! You will create memories that will last a lifetime and i'm sure that it will be a trip which you will never forget.
We love character meals and have recently seen Mickey Mouse at the following places:-
Chef Mickey's at Disney's Contemporary Resort
Ohana's Best Friends Breakfast at Disney's Polynesian Resort
The Garden Grill Restaurant in Epcot
Tusker House Restaurant in Disney's Animal Kingdom
I would recommend
Chef Mickey's for your first character meal. This is buffet style restaurant which serves Breakfast and Dinner. Although characters are always subject to change, whilst dining, your table should be visited by Mickey Mouse, Minnie Mouse, Donald Duck, Goofy and Pluto. Be sure to have your camera at the ready!
I would strongly recommend that you make an advanced dining reservation as soon as you are able to (up to 180 days prior to your visit) as this is one of the most popular restaurants and does get booked up well in advance.
Wishing you all the holiday of a lifetime!