Below are the basic things to fetching data to internal table.
DATA: IT_MARA TYPE STANDARD TABLE OF MARA, "internal table
WA_MARA TYPE MARA. "structure
"get data to internal table
SELECT *
FROM MARA
INTO TABLE IT_MARA. "will fill IT_MARA for sure if data present
"processing internal table
LOOP AT IT_MARA INTO WA_MARA. "will fill WA_MARA for sure if data present
WRITE:/ WA_MARA-MATNR.
ENDLOOP.
Hope this helps.