For this, I would just recommend watching and listening to other Youtubers that, either, you are subscribed to or are well-known.
When trying to improve on yourself, you have to learn how to analyze yourself objectively. Listen to what they say and do in their commentaries and notate what is different from your work; list what you need to do to make changes on to improve your faults or short-comings.
Another thing you can do is practice commentating your day to day behavior (narrate yourself) and make it sound interesting--as if someone is following you around and recording.
When doing any type of commentary--and you think you are not good--it is important to have some sort of script or cheat sheet in case your mind goes blank.
A lot of these things you really have to just go out there and learn on your own. People aren't going to hold your hand and tell you what to do. It's important to be able to improve yourself from within. You don't want to become dependent on others for your growth. If you want to be like the big Youtubers, watch them, learn of them, and emulate them in your own style never try fake (unless you are an actor).