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>