On the bottom of your modem router?
@Wmbro990 wrote: Security key
As stated, if it's a Comcast rental device, you can find it on a label on the back or bottom of the case where it might be called "Network Key" or "Password". Or at https://www.xfinity.com/support/articles/view-change-wifi-password.
If it's a retail device you purchased, we'd need the model number.
