hiya,
I am not sure which park you are going to so I will try to help a bit....
when you enter the parks the best way to find out where to meet the characters is by getting hold of a timeguide paper and guidemap which will show you the times when they will be around.
in MGM studios, we saw mickey, minnie, winnie, pluto ,goofy, stitch ,tigger near the sorcerers hat

on the hollywood boulevard.
buzz lightyear and jesse were at al's toy barn.
mickey in his sorcerers outfit is on mickey avenue,the street behind the hat.
that is who we saw there.
in Animal kingdom, we saw a few characters outside the gates, chip 'n dale , among others.
when you go to park minnie-mickey you will find minnie

, mickey and goofy there. donald was there as well but he was called one of the forrest friends.
pluto was to be found near pocahontas show.
Magic kingdom
main street square (right after you enter the park) is a good place to meet the characters. also in toontown. we saw buzz again in frontierland. you can also see ariel at ariels grotto in fantasyland.
we did not get to see any of the characters in Epcot.
I am also pretty sure we left out a whole bunch of places but maybe it is fun to find a few of those places and characters by yourself( that is half the fun of it ) have fun and enjoy the trip
hope this post makes any sense. greetings Pat