How do I log into an Xfinity (Comcast) router?

How can I log into the router and see internet usage and possibly sites visited?

I have a 15 year old grandson I am raising and he is on his notebook a good bit. I don’t want his curiosity to get him into something X rated.

I am not very tech savvy, but I am trying to figure this out. The home page comes up as login for gateway and I can not figure it out. Any information is greatly appreciated.