Do you have an agent? For the shows they sometime do open casting calls and they advertise with your states film boards, major papers etc.(if your are in California it's easier to find open calls).
But most of those kids get their auditions through their agencies. General auditions you do a cold reading of a script in front of a camera to see how you look and sound on film, then if they like you you will usually come back in for a second reading. As for the singing it really depends on the specific show. You will either fill out an informational form or have a resume attached to your headshot that tells them about any fun talents or hobbies you have (like singing, dance, insturments, skateboarding or whatever) and if they see something they want to explore then you address that with them!
This is super duper general I know but there are great books out there that can help you on your way. Good luck!
How do I know this? My younger sister is an actress and did Disney audtions