That is a HUGE question that I don't think can be answered in one post, but I can shed some light on your questions
"what content is needed?"
I'm not quite sure if you're asking what kind of content or how much content, but I will say one thing - movies & live web cams are very, very important. With more & more surfers getting on cable or DSL, they do expect to see moves & live feeds
"is it neccasary to have a company?"
No, but it is wise to have a company (and for many reasons other than having a paysite)
"how much would i need to start it?"
Cash? That all depends on what you can & can't do. You will need cash for one thing & that's content (regardless of if you buy it or shoot it yourself) If you can do graphics & whatnot, you can make the site yourself, so that's just an investment in time.
"what is the best way to promote it?"
The same way you promote other peoples sites - plus open a webm,aster program so that others can promote it as well