Anatomy of a press release
PR Newswire has a succinct diagram that highlights the elements of an ideal press release. Of the 11 key elements, to me the most important is the lead paragraph:
This is the single most important paragraph in the whole release. If you don't draw your reader in here, you've lost them forever. This is your chance to set up your story in a single sentence or two.
Obviously the headline has to grab the viewer's attention first, but the lead paragraph needs to convince them that you have something to say that will make reading the rest of the news release worth their while.
PR Newswire has a good FAQ page that gives an working introduction to the world of press releases (they call them "news releases.")
Please note that PR Newswire distributes news releases, but it is up to the newsmaker to write their own news release.