Cancellation of Gstin

I have a retail medical store and my turnover doesn't exceed 20 lakh in a year. But as business was registered under Vat it was the necessity to convert tin into gstin. Under Vat we used to give tax under composition scheme. Now I wanted to know do I have to continue to give tax under composition scheme under gst or I can apply for cancellation of gstin and not pay tax. Also sale of medicine are on MRP basis. please suggest.