If your region supports it, you can just enroll in the program from the phone’s settings. From there, navigate to Exposure Notifications and then select your appropriate state. The framework uses Bluetooth Low Energy to exchange a private key with any nearby phones using the same software. If you do come into contact with someone who has tested positive for COVID-19 (that person will have to report to the app stating as such), the phone will notify you that you may have been exposed.
In order for the Exposure Notification system to kick in, public health authorities for your region must agree to it. So far, the list of US states and regions that support it include Alabama, Arizona, California, Colorado, Connecticut, Delaware, Maryland, Michigan, Minnesota, Nevada, New Jersey, New York, North Carolina, North Dakota, Pennsylvania, Virginia, Washington, Wyoming and Washington, DC.