Comment by dirkdeman

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.