Hi Christine:
Congratulations on your first trip.
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.
If you have a particular character in mind, 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 information regarding characters that are typically found at the four theme parks.
Since this is a first visit, remember to check in at City Hall at the Magic Kingdom, or at guest relations at other parks, and pick up "first-visit" buttons. Many Cast Members will go out of their way to wish you and your party well, or greet you all with a kind gesture to make the visit more memorable.