There are several ways to do this, here are the top three secure Wireless Guest Access solutions:

1. The simplest method would be if your Wireless Access Point or all-in-one Router/Access Point provides a feature called Guest Access. If this is the case then you need only turn on this feature. 
2. If your Network Switch has a Virtual LAN feature then you could purchase and connect a second Wireless Access Point and have it broadcast on a different name (called an SSID) for your guests. The Virtual LAN will keep your guest separated from your office network, ensuring your safety.
3. Install a separate broadband connection exclusively for guest access.