There are no hard and fast rules. However, there are some basic guidelines you can use to maximize your chances that a publisher will notice you and your product. One of the best ways to contact publishers is through their web sites. Finally, the game industry has spurned a support industry of lawyers and agents who have relationships with publishers. However, if you use a lawyer or an agent to find a publisher, be prepared to pay a fee for using their contacts.
Publishers have different requirements for submissions from outside developers.
You should follow each publisher's submission guidelines to a tee. One false move, and your great game idea may never even be reviewed by the person with authority to approach you with a publishing deal. Finally, before you go through all the hassle of contacting a specific publisher, make sure that the publisher is likely to publish the type of game you are creating. If your idea is too similar to or too different from games the publisher puts out, it is unlikely that the publisher will seriously look at your proposal. By the same token, it is also unlikely that Talonsoft will consider your proposal to publish a tactical-level WWII wargame that will compete with their recently released title in the same genre, West Front.