You can select which fields to load from files or database tables and which views of the data source you want by using the interactive Select data dialog. The dialog shows different selection or transformation options depending on the type of file or database you use as data source. As well as selecting fields, you can also rename fields in the dialog. When you have finished selecting fields, you can insert the generated script code to your script.
You open Select data by clicking on a data connection in the data load editor.