You've always been great at your job. Over time, you have become known for handling more and more challenging work with ease, so much so that when something essential needs to get done, the higher-ups are confident in assigning it to you. This is where things have started becoming tricky for you, you are now being asked to be a public speaker at a company event, and you feel nervous about it.
The first thing you must do is realize that this fear of public speaking is not unusual. Some people have such a severely debilitating case of "stage fright" that they overcome their fears simply by seeing their therapist. More commonly, however, those with stage fright can overcome their concerns by changing how they think about public speaking.
You are undoubtedly nervous, but there are several good reasons why you should not be. Remember that the people who have asked you to speak are doing so because they believe in your capabilities -- this is a huge compliment!
Also, most people in the audience will be much more nervous about being in your spot than you are to be there. They will all wish that they were standing where you are, so part of them will genuinely believe that you are lovely and deserve to be up on stage.
Another reason is that people often feel less nervous when they know what's ahead of them. In this case, you already know everything that will be happening during the event. Therefore, you are no longer reacting to a state of the unknown. Your fear is based on what is known and can thus be reasoned.
Public speaking is something which you have done many times in the past. You may even enjoy it! Take some time to remember a time in your life when you have done something similar to the task at hand, perhaps making a speech for a class presentation or leading a discussion topic among peers. You will be able to draw on that experience and reassure yourself that you are ready for this situation as well.
Research about public speaking. Look up some famous speeches in history, and consider how the speakers managed to impact their audience. Try to emulate the more effective techniques they used while making your speech -- perhaps you will adopt a specific tone of voice or mannerism from one or another of these public figures. In addition, be sure to plan out your speech thoroughly. Practice in front of a mirror, with your friends, or even in the safety of your own home before you speak in public.
Remember that if you do not give your speech during this company event, the higher-ups will be disappointed in you. They might take you off of critical future projects. This is not about your public speaking ability; instead, it is a test of your ability to take on responsibility when it is presented to you. Public speaking may be outside of your comfort zone, but so are many other things in this world.