Locate the control panel for the alarm system. It will usually be near the main entrance to the home so the home owner can deactivate it quickly upon coming home. The system may have multiple panels throughout the home depending on the system configuration but any of them should activate the system effectively.
Look for the "On" button on the control panel. Many Brinks panels have it located in the upper-right corner.
Press the "On" button to fully activate the system.