What is needed in order to create a platform similar to youtube in that uploaded videos play commercials? I want a way to play videos online and while youtube offers a great way to do that I want to be able to sell ad space / commercials.
What is needed is a server-side program to run an ad video ("add" is math, "ad" is short for "advertisement") before the video the user asks for. You write that program, or you hire someone to write it. It's not something you can download and drop into your site, or have someone post here for you. (It's not even something someone can write for you without having complete access to your existing code, since one has to be written to use the other.)