Start by going to www.xfinity.com/id and looking up what your primary Xfinity ID is. Once you know it, you can do a password reset on it. You'll need to be on your home network if you don't have a mobile or email associated with the primary.
I am an Official Comcast Employee (On the HQ Identity Product and Serviceability Teams). Official Employees volunteer their time outside of their normal day jobs to create the best customer experience possible. If and only if you've been tagged, you can PM us by clicking on our name and clicking "Send a Message"