Most Helpful Guy
Do a bunch of stupid shit while risking your life, while becoming a really pathetic attention seeker. Oh, no, that's the wrong way to become popular on youtube.
I don't know of a single underaged famous youtuber. You'll have to wait until you grow some boobs.
But, basically what everyone else said. Those are all good tips. But really, all it takes is one viral video. It's more or less just luck. You can't become popular until a lot of people see your videos. And the only way that happens is if it goes viral. Like Jenna Marble's "How To Trick People You're Good Looking." She popped up huge after that one video. Also she's hot, so that helped.
Most Helpful Girl
Honestly, not trying to be a Debbie Downer, but the beauty and lifestyle category is overflowing with girls who basically have eaten up that niche market for all its worth. So unless you have something new or interesting to bring to the table it's not going to be easy to gain a really big following.
My suggestion would be to do things that other girls in that genre haven't done yet. Other than that, the dead horse has sorta been beaten to glue at this point.