There`s 2 things that i know will help.

1) breathing excercises. No matter how fast you can sing, if you run out of breath too quick, it won`t really be much good.

2) practice – once you got your breathing under control, try mimicking the guys you want to imitate.. Often.

It`s an oldie, but nevertheless – practice makes perfect.

It`s the routine you need to keep going.

It will take intense practice to become good at sight-singing. The following are some sites with extensive information:

Http://neilhawes.com/sstheory/sinterv2.h…

Http://www.ehow.com/video_2372359_sing-i…

I learn to sight-sing by associating specific intervals with music that is familiar to me.

As an example, the interval of a perfect fourth is associated with the opening notes of the Wedding March from Wagner's "Lohengrin."

I associate the interval of a tri-tone with the first two notes of "Maria" from "West Side Story."

I associate a minor-third interval with the opening notes of Brahms' Lullaby

Good luck on your audition!

I would warm up with a scale or exercise or some sort, try googling some singing exercises. Pick out a song that is within your vocal range (where you can sing without strain on your vocal chords, not too high or too low). It should have a simple melody and not super high jumps between notes. Singing from the chest is better than singing from your head, so think of your voice coming out from your lungs as you sing. At the performance, don't focus on the people there, try to imagine yourself in your room singing by yourself. It's normal to be anxious! Some more tips:
don't drink milk, sugar drinks, or thick drinks before singing because it will really clog your throat and mess up your singing
warm water is really good for singing, you might wanna try drinking that before singing

good luck!