You can get formal or informal education. Not only do you need to know how to create web pages and web sites (using html, javascript, css, plus a web page generator, like dreamweaver and optimize images using photoshop) but you need to know how to set up a web server, optimize it, set up security for it, know company standards for web development, copyright issues, and make sure the site is easy to navigate and use. The webmaster may need to oversee marketing, advertising, and order entry management.
More web teams have to also know how to manage databases such as Oracle, and use multiple types of programming (ASP.Net, C++, Java) and animation tools (Flash) to make their web sites noticeable to consumers and customers.
You can get the software yourself and teach yourself these tools and languages; buy books or review free internet resources, take continuing education classes, or take credit classes. The most expensive route is to pay a for-profit school. There still aren't a lot of certifications that are recognized for webmasters yet, but there is a Certified Internet Webmaster (CIW) certification you can get, but I haven't seen it listed in job requirements.
Good luck!