Are you trying to do this 'ad hoc" from FileMaker GO or to use FileMaker Pro to set up a script to export records via a predetermined set of specified fields for the export?
Thanks Phil for responding- the latter option.
I'm answering in terms of FileMaker 14. The details differ a bit for older versions.
Then in the scripts workspace, you select the export records script step. (Type in "exp" and select Export Records.)
Click the Gear icon and then select "Specify Export Order"
Either double click fields listed on the left or single click them and click move to add them to the export order pane. You can drag the small black double headed arrows up or down to change the export order. This will change the left to right order of the columns of data produced when you export.
You can also use the gear option to specify a field name and location for the file produced by the Export. You can even type in the name of a $Variable and use your script to specify the file path and file name for your exported file by setting the variable to that text.
For more info on this process see: "exporting data from FileMaker Pro" and "Export Records" in FileMaker Help.