Go to the alarm keypad and enter your arming/disarming code. Remember to press the "off" button once you enter your code. This turns off the alarm sirens and disarms the system.
Wait for the phone call from your security company. Most home alarm systems are monitored by a company. The security company calls to determine whether you have an emergency or a false alarm. Often there is a code or password known only to you and the security company. This is a means of identifying yourself to the company and alerting them to whether the alarm is false or an actual emergency.
Find out why the alarm sounded and take steps to resolve the issue. Most of the time, it is from a household member not entering the code correctly when entering or exiting the home. Reset access codes as necessary. Inspect the sensors throughout the home and ensure that all are intact. Call the alarm company if service is necessary.