Writing for the Web means writing for computer programs (search engines) as well as people.
Yes, you're a genius copywriter, but for people/ buyers to read your immortal prose, you have to get them to your Web page.
You can get them to your page via advertising, or via the search engines. Therefore, when you're writing for the Web you MUST learn to write for the search engines as well as people.
Start by creating your own Web site. I'd never hire a Web writer who doesn't have his own site, because the only way to learn how to write for the Web is to learn from experience. When you're in the trenches getting traffic to your own site, you're a lot more careful when you write for others: you learn to write for computer programs as well as people.
If you're a new Web writer, or even a pro, you may wonder what I'm talking about. Here's an example -- page titles. A page title (which appears in the browser) is a small thing. It's inconsequential to people, but vital to search engines.
Here's a great tutorial on page titles:
Nearly all experts in search engine marketing agree the most important element of an individual webpage is the title given to the page. This is the information that appears at the very top of the browser window when a webpage loads and within the underlying code (e.g., HTML code) that is enclosed in the element. From a search engine’s point of view, page title is the first indication of the contents of the page.
Page titles. Key words. The more you know about how search engines see Web pages, the better you'll get at writing for the Web.