To find articles about mass shootings, search in the databases linked on the Articles tab.
Good search terms include:
"mass shootings" or "mass shooting" or "mass shooters" (include the quotation marks)
"school shootings" or "workplace shootings" or other similar phrases
You can combine those with words about your specific topic.
If you are researching one particular shooting incident, you can search for it by the name(s) it is commonly given, for example:
Las Vegas massacre
Stoneman Douglas shooting or Parkland shooting
Don't put these in quotation marks, however, as they are sometimes called by other names and the quotation marks would be too limiting.