Hi Cecile -
Go to "Admin" at the top-right of your interface.
Once in the Admin section - on the left there is a link for "Field Management" - click that
Then at the top of that, in the "Fields" submenu, in the gray bar, there is a link to "Export Field Names" - this will provide an excel list for the REST API and SOAP API, as well as the Common Name for the field.