how to point to the next line of aninternal table

Hello experts,i am studying webdynpro abap, i am trying to point to the last line of an internal table, then add 1, in order to insert just in that position a new line by calling the function module ZFM_INSERT_CANDIDATE,neither the method INSERT_CANDIDATE nor the function module ZFM_INSERT_CANDIDATE  throws any error,.

The method testing just dont do anything. what could i try? the source is the following, :

method INSERT_CANDIDATE.

data: itab TYPE ztable_type,

t_line type zperson-id_person.

data: COUNTLINES type i.

"itab = me->SELECT_CANDIDATE().T_LINE = LINES( ITAB ) + 1.

DESCRIBE TABLE ITAB LINES COUNTLINES .

COUNTLINES = COUNTLINES + 1.

itab =  me->SELECT_CANDIDATE( ).T_LINE =  COUNTLINES .

   try.

     call FUNCTION 'ZFM_INSERT_CANDIDATE'

     EXPORTING

       id_candidate = t_line

       firstname = firstname

       lastname = lastname

       age = age

       telephone = telephone

       id_country = id_country.

     ENDTRY.

endmethod.

tnkx in advanced