On a cell phone, text 97520 in the message line and 888777 as the recipient to sign up for emergency (called "Alert") and "Advisory" text alerts only. If that's all you want, then you're done. You won't get anything via email or any "community" level messages, which are the least urgent.
MANAGING YOUR NOTIFICATIONS (Already registered)
Go to the Nixle Website to create an account and manage your delivery settings and pathways. Reset or retrieve your password if necessary (#2 below).