Before you first visit the alt.html newsgroup, you should be familiar with the posting standards in alt.html. For example, bottom-posting is the norm. Some people in the newsgroup get testy when people put their replies above the quoted text (these are helpful people, too).
You may want to get a decent newsreader. Many email programs, such as Outlook and Mozilla Mail, try to double as a newsreader. However, they tend to be clunky and inefficient to use. I suggest getting a program that is specifically designed for newsgroups. See the FAQ Newsreaders page to see a list of newsreaders.
You may also want to see What is alt.html
Another simple, good reference is at http://www.alt-html.org/ .
If you do not know HTML, or if you want to brush up your skills, try the list of HTML Tutorials.
When learning HTML, keep in mind that browsers display pages differently. Netscape, Internet Explorer, Mozilla, and Opera all display pages differently. It is a great idea to check your pages' appearance in all of those browsers. You might even like a browser so much that you use it for normal browsing! At any rate, see brucie's page for the a concise list of common, useful, testingworthy browsers.
A later note: