Typically, you might post a general announcement first. After that, to avoid irritating people, you might want to wait until opportunities arising in the newsgroup to mention your site in response to other postings. If there aren't any opportunities, you probably have no business posting to the news group anyway.
Some general news groups to try:
Comments to:
webmaster@geom.umn.edu
Created: May 22 1996 ---
Last modified: May 29 1996