Practice your music until you are almost tired of it: You should be able to sing your solo anywhere at anytime, even if you just rolled out of bed. This means paying special attention to the lyrics or text. Learn all of the right pitches (notes) and rhythms well and make sure you are giving special attention to any dynamic markings that may be in your music. Practice singing softly where it says p (for piano) and sing out when it says f (forte).
If you need to act during your song, practice telling your story.
Prepare a "practice recital": Grab some people you trust, some family members or friends to be your audience. You will need to practice your performance for them before going out on stage. You may be very nervous performing for your practice audience because you can see their faces and they are very close to you! But, you should relax because they are your fans! They want you to do well and they will be able to give you some good feedback about how you did.
Make your practice recital as close to the real thing as possible, which means you should walk on, bow and WEAR YOUR PERFORMANCE OUTFIT. Singing in your outfit will sometimes give you an idea that you might not want to wear that tight belt or that you stumble too much in those high heeled shoes while walking on stage. You don't want to find this out on the day of the performance!
Relax the night before the performance and get a good night's sleep. You may be very nervous and have trouble falling asleep. Try and relax. A cup of herbal tea or warm milk and a warm bath will usually do the trick.
Relax before the performance. This means, prepare well! Lay your clothes out the night before, arrive at your venue early, make a checklist, etc. You don't need more stress!
Have a wonderful time singing!