Understand the difference between SHYNESS & INTROVERSION. Shyness carries a fear of being judged negatively. Introversion is about how a person gets their energy or recharges. For example, an introvert may be perfectly capable of delivering a captivating speech in front of 5000 people, but will feel exhausted afterwards because they get their energy from being alone.
Know that what you have to say is valuable. No one on this planet has the exact same experience as you. You are more than capable of speaking and connecting with people in a unique way because you are just that-UNIQUE.
Practice: Record yourself on camera. Practice in front of the mirror. Speak in front of friends that will be honest and supportive enough to give you constructive feedback.
As an introvert myself, I know how difficult it is to get comfortable with public speaking. Here are some tips that helped me get more comfortable with public speaking:
Realize that most people are in the same boat and are nervous about public speaking. Fear of public speaking is one of the most common fears out there!
Preparation will build your confidence. The more familiar you are with your speech/presentation, the more comfortable you will be. In law school I would write down my oral arguments and practice them out loud in front of the mirror until I no longer needed the paper in front of me. This helped me feel more confident when I was up in front of a group.
Before presenting, try a breath exercise (especially if your voice tends to get shaky). Not only does this help with your voice, it also relaxes you.
Breathe in deeply, filling your lungs.
Hold for a moment (do what feels good for you but a few seconds does the trick).
Release the air in your lungs through your mouth.
Hold for a moment before you inhale again (a few seconds is enough).
Repeat 3-4 times or as much as you feel you need.
During your presentation, try to focus on one person at a time as opposed to the entire audience at once. Starting out, you may want to find some friendly faces in the group and rotate between them so you are not focusing too much on one person. I used to find those people with reassuring smiles and focus mostly on them during my presentations.
If you make a mistake or forget what you were going to say, pause for a moment and carry on with what you remember. A pause will feel longer to you because of the adrenaline, but a short pause to collect your thoughts can seem intentional (and also have a dramatic effect). The audience does not know what you planned to say that day. If you skip a sentence or two, the only person who knows that is YOU- so don't worry about getting the words "exactly right".
Do your best to put yourself in situations where you are practicing. Start out small and continue to push yourself toward opportunities. Often times through school you will get the opportunity to present a project- look at it as practice. If you do not have as many opportunities in school, sign up for a public speaking class or similar program at a local college or high school. The more you practice the easier it will get!