There are some norms for website content article writing, and there are guidelines that are specific to each given website. For example, writers guidelines for submissions to Writer's Digest includes discussion of philosophy and readership meant to guide your focus as well as details pertinent to each type of article they publish. So your first step in getting started in writing an article for the Web is to familiarize yourself with the writers guidelines of the website(s) you are targeting.
Some stylistics relevant to website content writing have developed over time. Such things have become stylistically preferential as single space between sentences, not double space; use of subheadings; and top-loading content to concisely tell the whole story in the earliest lines of text in case readers stop reading and click away from the article or site early.
Spelling style has evolved as well. Beginning as "e-mail," it became "email," but is now "Email." Once "Web Site," it is now "website," but "Web." The "Internet" has always been and remains the "Internet."
So the second step in getting started in writing an article for the Web is to absorb and learn the style that is electronic-style, or e-style. AP style (different from APA style) has developed much over the years by way of incorporating Internet writing style into its standard stylebook, and many websites, like digitaljournal.com, use AP down style for their titles (only the first word is capitalized).
As to article structure, studies have shown that readers of Internet content read in a F-shaped pattern. The first few lines get a lot of reading attention but then the focus shits to the right and scans. If nothing compelling deepens the reader's interest, the focus floats quickly over material on the left and their reading is finished, though a lot of attention is given to side bars or text fitted to the right or left around an image.
What this means to you as a writer is that the crux of your article should be presented concisely in the first few lines. It also means that any words, concepts or sentence syntax that might stop a person dead in their F-shaped reading tracks be avoided by placing detailed information lower in the article and compelling, interesting information in upper prongs of the F-shaped space.
So your third step in getting started in writing an article for the Web is to master the specifics of F-shaped pattern reading and to accommodate the development of you article to this pattern, remembering that if interesting and compelling information is presented in key areas of the F-shaped pattern, then readers will stay with you through the whole article.
First step is to figure out a topic for your article. Based on the fact that it is an article, it will likely be an informative article or a persuasive topic piece.
In either case, you will need to provide factual information to your reader. If you are writing an informative piece, leave as much personal opinion out of it as possible. You want to stay as neutral as possible. Remember that you will be publishing this to the web, so author bias needs to be kept minimal. If you are writing a persuasive topic, then your opinion as an author matters. Your task is to convince your readers that your opinion is the correct one. This is best done by using the support of factual information and providing sources. No matter what, proofread it. Don't rely on spell and grammar checks. It will be published as a document, so you want it to be as polished and professional as possible.
I think the easiest way to publish a document to the web is to use Google Docs. Type your document on Google Docs. Then go to the "file" menu and choose "publish to the web," then click "publish."