Hi Jessica:
Disney World has some
general information online regarding character greetings, including the popular
Epcot Character Spot.
When you get to the parks, pick up a complimentary Times Guide. Along with providing information regarding show times, parades, and special events, the Times Guides generally include character greeting times and locations.
There are several "holy grail" characters that are hard to find. I ran into one of these characters, Goofy's seldom-seen son Max, at
Cape May Café during my last trip. If you have a particular character in mind that you'd like to see, your best bet is to stop by guest relations at any of the theme parks and ask them to call the "character hotline" for you. The character hotline will be able to tell you if, where, and when any particular character will be in the parks that day.
Another resource is to visit website allears.net (not an official Disney site) and use the search engine to search for "characters at a glance." You can find more information regarding characters that are typically found at the four theme parks.
Happy character-hunting and enjoy your vacation.