How do I download data in my internal table in a CSV file?

Use the Function Module SAP_CONVERT_TO_CSV_FORMAT to convert the internal table into Comma seperated format then download this internal table using the Function Module GUI_DOWNLOAD.

See a sample program below:

TYPE-POOLS: truxs.
TYPES:
 BEGIN OF ty_Line,
   vbeln LIKE vbap-vbeln,
   posnr LIKE vbap-posnr,
 END OF ty_Line.
 ty_Lines TYPE STANDARD TABLE of ty_Line WITH DEFAULT KEY.
DATA: itab   TYPE ty_Lines.
DATA: itab1  TYPE truxs_t_text_data.

SELECT
 vbeln
 posnr
 UP TO 10 ROWS
 FROM vbap
 INTO TABLE itab.

CALL FUNCTION 'SAP_CONVERT_TO_CSV_FORMAT'
 EXPORTING
   i_field_seperator    = ';'
 TABLES
   i_tab_sap_data       = itab
 CHANGING
   i_tab_converted_data = itab1
 EXCEPTIONS
   conversion_failed    = 1
   OTHERS               = 2.
IF sy-subrc <> 0.
 MESSAGE ID sy-msgid TYPE sy-msgty NUMBER sy-msgno
 WITH sy-msgv1 sy-msgv2 sy-msgv3 sy-msgv4.
ENDIF.

CALL FUNCTION 'GUI_DOWNLOAD'
 EXPORTING
   filename = 'C:\TEMP\test.txt'
 TABLES
   data_tab = itab1
 EXCEPTIONS
   OTHERS   = 1.