The human eye sees everything Horizontal, like a big TV/movie screen, so most likely, most of the pictures taken should be in a Horizontal format.
If you are shooting in a Photojournalistic Style, you want to shoot always Horizontal and generally with a wide angle lens. The Photojournalistic Style documents a certain story, you need to show a subject surrounded by elements that will “tell the story”. The elements can be a variety of things depending on the location and situations, but all these elements are to be supporting/complementing the subject. On the other hand, if you are shooting Vertical, that format will not give you the versatility to show those elements around the subject (again, with some exceptions) and you’ll miss the point.
Always try to include something to support the subject and don’t leave any blank spaces that won’t tell you anything relevant.
Keep in mind, there are always exceptions to the rules.