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.

I agree with dirkdeman, you can't start a big web app overnight - you start small and learn by doing. Try deploying on a VPS and when you get stuck, you can ask http://serverfault.com. Also I would recommend checking http://debian-handbook.info/ for reference (if you're going to deploy a debian based distribution). Good luck!

Just do it, + google. I can't think of any better way. If you're leaning towards Red Hat/CentOS, then an RHEL Certification book will fill in a lot of knowlege holes. Always keep a Amazon AWS micro-instance or a local Linux server to mess around on.