Turn over limit under gst

Our Government announced for Registration Under GST Act Turnover limit Rs. 20 lakhs. If A Assessee Have turnover 0 to 20lakhs How to he/she transact his business...? How to made Invoices...? How to answering to the AO on road side Squad...? and How to shown he is the Under 20l business person. I mean any certificates or any approval for the business...?P lease reply madam., This is one of my Doubtful for Busines Turn Over limit....