I am hooking up a new computer and it is asking for the network security key. I am not sure what that is.
Sounds like the WiFi access security key. If you are connecting your computer directly to your router using an ethernet cable, you don't need WiFi and could turn off/disable the WiFi in the computer. If it only has WiFi access, the you'll need to get the network security key from your gateway/router providing the WiFi signal.
If you are using a Comcast gateway, see https://www.xfinity.com/support/articles/view-change-wifi-password. If you are not using a Comcast device please post the model number of your WiFi router or gateway.