Below turnover 40 lakhs gst payment?

Sirs,

My doubt

if the turnover of a small business man not exceeds Rs. 40 lakhs in a financial year. 

• which type of books he has to maintain?
• if he is registered under GST, what to do? (i.e. is he pay the GST or not and what about GST returns)?
• if he is not registered under GST what to do ? (i.e is he not to pay any GST and submit GST returns?)

Please clarify my doubts.