How to add custom field in invoice

Hi, everybody, I want to add a custome field for GST(Goods & Service Tax) Number just below my business address. So that every invoice I create will have my GST number by default. Can anyone help me on that?. I would remind you I am not a person with php knowledge.

thank you.
Mahesh Palamuttath
Kerala, India
mahesh@opensio.co.in