The first thing you want to do when preparing an audition is read the instructions carefully. Are they asking for a monologue? A song? Or will they provide a scene for you? Do they want you to label your audition a specific way? Casting directors see hundreds of people daily, and not following their instructions is a sure way for them to dismiss you and move on to the next face.
Second, do some research. What kind of project are you applying to? What's the style? If it's a play, read it. Look up more work done by the director, the playwright, even the casting director. That way you can get a sense of what they like and look for.
Now, for the actual audition piece. Read it, and see what information it gives you. Who is your character? What are your relationships like with the people around you? Identify what you want, and what is standing in your way.
Extra points if you memorize it fully and can do it without holding a piece of paper. And to really wrap it all up, you'll want to find a nice button at the beginning and end. Start acting before you say your first line, and don't stop acting as soon as you say your last line. What is the moment before? What happens right after? Let them see you live in those moment for a bit. Do something that isn't in the script. Something that's your own.