To watch through Twitter, people should tap or click the heart (like) button for this tweet, which is also embedded below. Apple will then send out updates on the 12th, including instructions on how to watch.
Otherwise, the easiest way for most people to watch will be via Apple’s event page. A video stream should appear shortly before the event starts, though the company’s preferred streaming technology means only certain browsers are guaranteed to work. This includes Safari on macOS 10.12 or iOS 10 or later, and on Windows 10 PCs, Microsoft Edge.