Final answer:
A first responder should secure the crime scene, ensure safety, and assist the injured while preserving evidence.
Step-by-step explanation:
Upon arriving at a crime scene, a first responder should prioritize securing the scene to ensure both the safety of those present and the preservation of evidence.
This entails establishing a perimeter, potentially with police line tape, to control access to the area.
The initial assessment of the crime scene involves scanning for any immediate threats or risks as well as providing or coordinating the necessary emergency services, which can include administering first aid if there are injured persons. This primary assessment must be done carefully to avoid contamination or loss of evidence.