What would be a good written introduction for the topic of gun violence?

To write an introduction of this topic, you could start by sharing some statistics on gun violence. You could focus on your country specifically. For example, according to the Gun Violence Archive, there were over 61,000 incidents involving a firearm in the US in 2017. Using the charts on this website, you could compare and contrast the number of gun violence incidents over the last decade. (See the first reference link provided).
Alternatively, you could compare and contrast the number of gun violence incidents among different countries. See the report in the second reference link for some statistics.
Another option is to focus on a specific incident of gun violence. You could use an example which happened in or around your home city, or something which made the national news. You could talk about what happened in this particular incident, who was involved, and if any people were injured. You could also discuss the incident's aftermath, such as how many people were injured, or if the perpetrators were prosecuted.
Once you have included one or more of these ideas, you can then move on to the main body of your essay.
http://www.smallarmssurvey.org/fileadmin/docs/U-Reports/SAS-Report-GVD2017.pdf