Make a Page
To being with you will need to first create a page in order to have something to designate as the Canvas front page. Once you have created a page you will need to PUBLISH THE PAGE (if its not published you won't be able to select it). After you have create dthe page click on VIEW ALL PAGES...
View All Pages
When viewing all pages select the opage you wish to designate as the front page and click on the little gear at the far right. Select Use as Front Page.
Once you have done this you lwill be able to tell Canvas to use the page you specified as the Fornt Page