The most important initial step after installing a new wireless router is to change its default password to a strong, unique password. Default passwords are well-known and easily discoverable by potential attackers, who can leverage these default credentials to gain unauthorized access to network devices. By changing the default password, you reduce the risk of unauthorized access to the router's administrative settings, which could compromise the entire network. It is not sufficient to leave the password as default, nor is enabling firewall features a priority over changing the default password, since the router can still be managed remotely with the default credentials. Updating firmware, while important for fixing security vulnerabilities, does not directly protect against unauthorized access due to default passwords.