GST for startups

Goods and Service Tax is applicable to all Indian service providers(including freelancers), traders and manufacturers, once their supply turnover crosses Rs. 20 lakh. The GST is an all-in-one tax that subsumes a variety of state(VAT, Entertainment Tax, Luxury Tax, Octroi) and central taxes(CST, Service Tax, Excise Duty).

If you are VAT user, then you can check this link: GST Registration Process for Existing Gujarat VAT Dealers

For full details of GST, check this source: GST Applicability and Registration Details

Yes you can opt for GST voluntary and start your business as a registered dealer and avail all merits of a registered tax payer.

to sell online via amazon you need GSTIN even to register