Hey, I am a fresh PhD student, and was asked just this week to teach today a class of almost 50-80 Students...I really want to pursue a teaching career and love to explain things to people, but only if I am confident enough about the topic. However , such large amount of student seems threatening to me and the topic is also somehow new to me ......and the biggest problem I am naturally a very introvert person, so I am absolutely afraid to start stuttering and talking nonsense, making grammatical mistakes while talking... I know if I mess this up today, there is no chance for me to have another opportunity to prove myself... I don't know what kind of advice might help me. I really needed to get my thoughts written down...and see if others with similar personality have overcome their anxiety disorder..

I will update you after class and tell how it went..

I usually direct myself inwards when I do the presentation and I focus on the presentation itself, like I'm playing in a movie or a school play. I don't look people in the eyes, just at the audience like it's a blurred image. To the outside observer I seem confident and outspoken, I decompose only after I've finished and I have time to think about the performance. It's similar to a lot of people, I guess.
I had clinical panic attacks and anxiety disorder, and still did presentations then. I'd rehearse the whole thing at my home before, several times, and two days before. Then I'd behave like a tape recorder. It worked. (anxiety disorders eventually dissolve when treated behaviorally).

Thank you for your reply! I am actually good at given presentations. Because I always prepare myself perfectly. But this is not a presentation, this a two hour class, where I have to interact with students, and go through exercises and explain and answer their questions...

I'd prepare as much as I can and if I don't know something, I'd just say that I don't know/remember, but I'd direct them to the source (for example, a book). And pretend that I'm explaining it to my little brother. It gets better with habit. Anxiety tends to disappear when something is really well rehearsed.

It went so well . First I was nervous , but as soon I realised that these students know so little ... I was really confident , tried to encourage everyone to participate ...which did not really work , same 5 students kept answering and talking , I made sure to give really positive comments on their answers.

. And really enjoyed explaining things on the board . I was so surprised by myself and looking forward to do this again.

So if you ever found yourself in this situation .. remember these are students , who know for sure less than you , emphasise on the topic your confident to explain ..

Good for you! : )