Obtain a talent agent, if possible. While some stars are "discovered" or get their big breaks through open casting calls, many roles, and even auditions, are closed to performers who are not represented by agents. Try to make contact with as many agents as possible -- invite them to see you in performances, and talk to them about the clients they represent and the type of roles they generally cast.
Audition for theme park and cruise line roles. Auditions for singers, actors and other performers are held regularly all over the world. Check the Disney Auditions website for upcoming auditions and locations. Singers are required to prepare two songs in contrasting styles and to select 16 bars of each. You should bring sheet music (in your chosen key) for each, with the selected bars marked. A pianist will be provided. Be sure to bring a resume and head shot.
Audition for Broadway and Broadway-style productions. Disney has now expanded its stock of Broadway productions (including touring companies of these same shows) to include numerous shows, such as "Beauty and the Beast," "Mary Poppins" and "The Lion King." Auditions are less frequent for these shows than for theme park roles but are also posted online at the Disney on Broadway site. The requirements for each audition vary, but you should be prepared to sing in different styles and should bring your resume and head shot.
Audition for Disney Channel productions and Disney Films. The Disney Channel (along with Disney XD) and the various Disney film companies regularly hold open casting calls and talent searches. The requirements for each vary. Keep an eye on your local trade paper for local casting calls, or visit one of the many sites online that track upcoming Disney auditions.