Hi Kiran,
Texts are generally determined in your invoice based on text given in your sales order. If you give text in your sales order (manually), then it will get copied to your invoice. Now to get this text print in invoice, you need to write a logic in smartform (output program) from your technical person based on its object (VBBK) and test ID (which you customized in VOTXN transaction).
Else, Go to transaction SO10, create the Text, then use that text name along with your billing type (ZF2) in the smartform (output program).
Regards
Shailendra