using append,insert statements u can enter the data into the
internal tables.

u can use,

SELECT F1 F2 .. FN FROM <DB_TABLE> INTO CORRESPONDING 
FIELDS OF TABLE <INTERNAL_TABLE>.

as well.

There are 2 ways of populating an internal table 
1. From Database Table 
2. From Work Area of the same line type

1. SELECT <field 1>, <field 2>, ... <field n> 
   FROM <db table> INTO table <internal table>
   WHERE <condition(s)>

2. INSERT <workarea> INTO <internal table> INDEX <position-
to-insert>

OR you can use append to insert ... APPEND <workarea> TO 
<internal table>