I know how you feel, i get nervous in auditions too. What i do is try to relax and take a deep breath before singing. Maybe image your the only one in the room and no one is watching. Your posture should be straight….. And don't forget to smile and show a confident attitude!!!
For sight reading, i know, difficult, but there is a way to practice it. You can find pieces that you've never sung or have heard of before, sing them (maybe record yourself??) and then play it on the piano if possible, since you said you can do that well, and then fix your mistakes. PRACTICE MAKES PERFECT!!

Good luck out there!

When you are going for an audition, make sure to dress nicely and to not wear shoes that are too high as they can alter your balance and affect the way you breathe due to posture changes. Always keep your arms relaxed at your sides, make sure you are not lifting your chest while breathing (indicative of a chest breath and not a diaphragmatic breath), and do not stare at the person you are auditioning for too hard! It helps to glance over, and then across the room at your "imaginary" audience. Make sure you know your material very well, as losing the words is one of the worst possible things that can happen in an audition.

Make sure you are very well warmed up before you go into your audition, and try to free yourself of any nerves beforehand so you don't crack up infront of them. Be confident, smile, take their response – whatever it is – graciously. A good attitude goes a long way. Good luck!

I'm doing a singing audition tomorrow, and I looked up some singing tips. First, you should bring a water bottle with you to keep yourself hydrated. You should also drink herbal tea with honey, and avoid all dairy products the day or days before a performance. About 2 hours before your audition, you should eat a medium sized meal, so you're not starving but not really full either. Eat foods like eggs, apples,and pasta. Don't eat chocolate, cold water (or any cold drink), soft drinks, or acidic things (like citrus). Also don't eat any junk food, even if it's healthy. (like vegetable chips, etc.) You should gargle salt water also. (just be careful. I tried doing that today, and when there was only a few drops of water left in the cup I poured it all into my mouth. Apparently all of the salt built up at the bottom of the cup and I got a disgusting bitter mouthful of salt and almost threw up) You should also chew gum right before singing so your mouth is moist, and don't clear your throat.
And if you're really serious about becoming better for this audition, you can buy a Vicks steamer for about $35. I'ts kind of like a humidifier, but breathing in the steam, (according to the direction), will help your vocal chords.
Hope this helps, and good luck!!!