Most of writing for the Web is just writing. If you aren’t writing much now, remember what you learned in school: most successful writers read widely, write regularly, and publish what they’ve written frequently.
Read Web pages more thoughfully than you normally do, since you’ll now be looking to learn both style and facts from them. Notice how paragraphs are short and direct, because that looks best on the screens we see the Web on. And key phrases will be links to pages that amplify the story you’re reading.
Apply this insight to your own writing:
You will need to write more than you think going in to get really good at it. I’m not there yet myself, either.
In 2016, basically any site builder, like Squarespace or the Blogger post editor, will make HTML that’s good enough to get your point across. You don’t need to learn HTML when you’re starting out.
This is a good thing. When I tried to learn both HTML and writing for the web at the same time, I found it was way more comfortable to memorize details of HTML than actually write anything. And even though it became my job, I regret not having written those stories from my freshman year of college.