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.