GST Registration for online Grocery Store

Hello!

I want to know whether GST registration is required to start a retail online grocery store to sell fruits, vegetables, household needs, body care needs etc to serve the requirements of a small area, say a town. It will operate within 30 km range, within a district, within a state. In NE India, where the threshold limit is 20 Lakh. Can you please explain how it will work, which law is applicable. What is TCS ? Whether it is applicable to it ? Very confused ? Please help me out.

Thanks.