In public speaking or like in everyday life. For me its all about sitting down and really looking at myself in the mirror, and reminding myself that I am who I am , and no one can change me. No matter what any one else says I am me. People talk about others all the time but as long as I know who I am and truly love myself, other peoples opinions don't matter at all.
Its also about putting yourself out their, the more you do it the more confident you will become. Like making it a point to introduce yourself to new people instead of waiting for them to come to you. Starting a youtube channel, can be a good way to gain confidence, when I did it at first I was scared but the more I made videos, the more confident I got, in life.
Lastly always be yourself, no matter what, don't change for anyone. Cause if someone wants you to change they don't really like you for you.