The first step in securing a newly installed SOHO (Small Office/Home Office) wireless router is to ensure it has the latest firmware installed. Firmware updates often include important security patches and new features that protect the router from known vulnerabilities. Other actions, like disabling unused ports or guest accounts, can be useful for hardening the security, but they should come after the router is running the most recent and secure version of its firmware.
References:
CompTIA A+ Core 2 Objectives, Domain 2.1: Best Practices for Securing a SOHO Network (ProfMesser) (Whizlabs)