If you're asking a question like that then it's a bad idea.
You will need:
* 1 or more grunty dedicated servers, insane amounts of disk space and bandwidth;
* a script to handle your uploads and file management, and a well-coded and secure one, so that excludes most of your shitty $10 an hour freelance coders;
* a lawyer to handle all the copyright complaints you're going to get;
* a full time employee to monitor uploads for ch*ld pr0n, terrorist shit and other highly illegal stuff.
It's a shitty way to earn unless you're rich to start with, because it's a very low margin business and you need heaps of traffic (therefore you need to use HEAPS of bandwidth and storage) to get any decent income out of it.
A lot of filesharing sites (eg megaupload) are off shoots of tube sites. They don't make that much on the hosting side of things but they use the added volume to negotiate better deals with their hosting providers. In some situations, a filesharing or tube site can push so much bandwidth that it enables their upstream provider to negotiate a cheaper deal for their wholesales bandwidth, and the saving gets passed on to the filesharing site.