clearing internal tables and work areas

hi this is my sample code can u tell me where and how to put my code for clearing and refresh work areas and internal table!! i have never used them before.

CLEAR gi_cstctr.

REFRESH gi_cstctr.

SELECT hkont

FROM bseg

INTO

TABLE gi_cstctr

WHERE

kostl = bseg-kostl AND prctr = bseg-prctr.

IF sy-subrc EQ 0. "if rows are returned

SORT gi_cstctr BY hkont. "sorting the internal table.

ENDIF.

*if cost centers are there in the internal table then loop the internal table into corresponding work area and if the G/L account matches any costcenter in the table

CLEAR gw_cstctr.

IF NOT gi_cstctr[] IS INITIAL.

LOOP AT gi_cstctr INTO gw_cstctr.

IF gw_cstctr-hkont EQ lv_hkont.

lv_flag = gc_x.

EXIT. "set the flag to x

ENDIF.

ENDLOOP.

ENDIF.