When an aircraft is created, you would normally set the current engine and airframe hours. This provides a base figure for the airframe hours. Note for aircraft fitted with an hour meter, this usually records the engine run hours (Tacho).
If you were unable to set up the initial hours when adding the aircraft you can add a new service to correct for this. Just navigate to the aircraft's log tab and add a service, first for the engine and again for the airframe. In the notes section, we recommend stating the date of the last service for the first service entry. See Recording Servicing for details.