Click the View My Channels link in the upper right corner. It will take you to a printable channel line-up.
Also: Comcast may have included a lineup with one of your recent bills. Check https://customer.xfinity.com/.
Service Centers and Xfinity Stores should have printed channel lineups. Check locations and hours at http://www.xfinity.com/support/service-center-locations/ to pick one up, or call Customer Service at 1-800-Comcast and have them send a lineup to you.