You can create a new family tree by clicking on the "My Account" button in the upper right. Near the bottom of the account page is a button to "Create or import new family". This will take you to a new, blank family tree. You will notice at the bottom of the left sidebar a button that says, "Import GENCOM or Family Script...". Click that button and import your file. Note that it has to be in GENCOM or FamilyScript format.
