Hi Kanchana,
Maintain check boxes for all fields as per the requirement in selection screen.
Read all the entries of the table in to final internal table. While preparing the field catalog ( I assume you are using classical ALV methodology rather OO ALV ) for the screen output , for each field in the table check for the condition whether it is checked in the selection screen then only populate that field in field catalog otherwise ignore the field.
So intern , the catalog populate only required fields.
Reward points , if this helps.
Regards,
Subbu