As others said, practice.
I feel like "practicing" is something a lot of YouTubers do not want to do. Like the idea of not buying a camera and 20 minutes later you're on YouTube is just completely unacceptable. Remember, you're putting yourself out there for the world to see, and if you want to make a good impression you need to know what you're doing.
If you're not comfortable performing or speaking on camera, then all that's gonna fix it is doing it a LOT. There really isn't like some secret method that'll boom make you able to speak naturally and effortlessly. It's like learning to juggle. People can tell you tips and hints to help along the way, but unless you put in the time you won't get it, and just like juggling you probably don't want to step on stage until you're ready to.