You don't simply start one, even when you have enough starting capital to do so, unless you want to go ala 'AwesomenessTV' where you are going to be a d**k to every partner and only partner them to gain money, as most of the channels won't even reach the minimum payout and such. Also, you will need a pretty organized team with people fulfilling support, PR , recruiters, people with uber high knowledge (and most likely degrees with legal issues and what not) regarding copyrights and what not (so not your average educated YouTuber and such).
It's isn't something started like BAM if you want to do it properly.