You learn by doing, reading tutorials on the internet, by asking questions on StackOverflow and by making mistakes. Start out small with some basic web hosting, or play around with a VPS. Get comfortable with using Linux. Or tinker around with Amazon Web Services (free for the first year!)But really, don't worry about scaling too much. You don't just start a big web app, even Facebook took a couple of years to get to a size where they had to worry about scaling.