Like StupidQuestions345 said, believe in yourself.
If you don't believe in yourself, then you will end up giving up, and you won't try. If you do believe in yourself, then you won't give up, and you will try your best. If people say you're amazing, then you are.
When it comes to singing, it helps to touch the roof of your mouth with your tongue, when you're singing high notes. When you're singing low notes, touch the bottom of your mouth with your tongue.
Before you start to sing. Inhale deeply, then exhale. Do this 3 times. Inhaling and exhaling helps you relax your vocal muscles, as well as helping you relax when you're nervous too.
When you step on a stage, with a huge audience, just smile and say to yourself, " I can do this." Never say, " I can't do this." Loosen up and be yourself when you sing.