You don't need too much opening remarks, just introduce yourself and give the judges an image. When trying to speak, you should pay attention to natural posture, fluent language, prominent focus and accurate expression.

Generally, the first few sentences are: Hello, I am xxx, and the main content I want to talk about today is XXX. I hope you can give me more advice. Now let's begin our class.

If you just graduated, or have no teaching experience, the main test of lectures is your ability to express and organize classes. Pay attention to these two points, there should be no problem. Wish you success!