Newsmakers themselves sometimes notify the media. Press releases (usually) fall into this category. As you might suspect, this type of news is often as much advertising as news, but that doesn't necessarily mean it isn't newsworthy. Events calendars typically contain this type of info as well.
Regular sources of news stories (police, courts, other governmental agencies) often have a designated contact person or place from where they disseminate information whenever there is any. The media regularly look for news from these designated sources.
The vast majority of news comes from other news outlets, over what they call "the wire". The Associated Press (AP) and United Press International (UPI) both operate newswires that constantly feed news from their affiliates located all over the world. Newspapers, and to a lesser extent, television news channels, subscribe to these services and pay for the right to reprint or report items from it.
