You can try to give yourself hints, telling yourself "I can do it", "I am fully prepared", "improvise" and so on. If you are nervous, you can try not to look at the audience below at first, just like you are preparing a lesson and talking to yourself.

In fact, nervousness is a normal emotional response. Everyone will be nervous when he speaks alone, because he will show himself in front of many people and will be afraid to show his bad side. Even the most experienced lecturer will be nervous when he takes the stage, but this does not affect his teaching, just as we always have happy and sad times, but it does not affect our normal life. If you can, try to speak nervously on stage. Don't care about this "nervousness", just think of it as uncontrollable physiological reactions such as accelerated heartbeat and sweaty palms. As you talk, you will find yourself immersed in the speech, forgetting your nervousness for a long time, or getting acquainted with the audience, and you will feel like chatting with friends.