That's an ambitious project if you don't have a clue about the web world. It also depends if by watching online movies you mean selecting videos in youtube and showing them to your viewers or you're thinking on a subscription site (like netflix) where people can watch movies and series or anything that they may watch on TV.
So the very first thing, you need to know what you want and be specific.
Especially if you're going to monetize on this, you'll need a business plan and that means doing your homework properly: who will watch this?, which is my market?, how many of them are? How to get the content? Copyrights? Expenses: hosting, electricity, promotion, employees...
You also need agreements with the companies that distribute the movies or with those who make them so you can have access to their movies and permission to play them online.
For the website itself, you'll need a good designer, a good programming team and a good hosting (lots of bandwidth and reliability as well as a hard disk space, a cloud system with backup copies and redundancy will be better)