Judge the interest level. Consider if your audience is likely to pay attention to the subject matter. Determine whether the subject touches home with them, or helps them in their daily life. Decide if the story relates to a recent or a current hot topic.
Consider the impact. Examine whether the story affects a significant portion of the audience. A story that isn't important can still lead the TV news as long as it's interesting. But you're more likely to keep the viewers attention if the story affects them in some way. Often the challenge is to show the audience why they should care about a particular story if the connection isn't obvious (such as a government action that could lead to higher taxes).
View the video. TV news depends on video, so when choosing the lead story you need to consider the type and quality of the pictures. Dramatic shots of a burning fire can catch people's attention more than talking heads at a government meeting. In lieu of video, strong graphics can help with a lead story. But graphics likely won't be enough to move a story into the first position, while excellent pictures can be a deciding factor.
Live for live. An on-scene reporter can greatly enhance the interest in the lead story for TV news, so having what they call a "live shot" can be a good reason to put a story first. A live reporter can also help explain why the audience should pay attention, or conversely signal to the viewer that the story they are watching is worthy of their interest.
Compare the competition. TV news is a competitive business, and you need to give your viewers a reason to watch your show instead of someone else's. If you have an exclusive story, you'll want to feature it prominently in order to call attention to your strengths.
Make it fresh. If a story is old or already reported in a number of other media, it's less likely to keep the viewers interest. But if it's a breaking story be sure you are among the first to report on it. That makes a story a better candidate to lead the TV news.