The best way to start getting ready for an audition is by reading the play or listening to the music for the show you're auditioning for. Avoid watching videos of the show online, as this can greatly change how you first approach the material. You want to make choices that come natural to you - not belt a note a specific way because "that's how Sutton Foster did it".
Next you should select an audition piece that is age appropriate and similar to the show. Usually the casting director will specify if you need to come prepared with your own monologue or a piece from the play. Sometimes they will provide specific selections from the show for you to read/sing from. These are called cold reads. It is crucial that you are familiar with the show before walking in, in case you are randomly asked to read for a specific character. You don't want to end up making an ill-fitting acting choice in a scene, such as grinning through the eulogy of a deceased relative.
If you have never auditioned for a show before, practice in front of friends and relatives. Have them "peer review" your performance. Ask them what they think the mood of the piece is, what kind of vibe you give off, etc. This will help you get an idea of what others perceive you to be while acting, so you know if there are any changes you need to make with your voice/stature to make your acting choices more believable.