GSTIN number is necessary from July onwards to sell stuff online. You may have to approach CA or the online agencies to register yourself as sole proprietor or under some name. When you are selling physical products, check with the GSTList if your product or service is exempted from the list. Considering it is clothing it is not likely to be excluded unless it is khadi work which is considered in exempted. You can't sell this product online without registration.
Flipkart, amazon, ebay, shopify all require you to register under some brand name or sole proprietor. Here you may be asked to register for GST and VAT. After July 1, the guidelines will change on almost every E-commerce platform. I suggest you to get in touch with any specific site like flipkart or amazon and ask them the changes that applies. You can wait upto August for things to properly settle and then decide on strategy considering the rules and the regulations are likely to stabilize from there onwards. Till then you can sell offline in nearby stores or in flea market where your GST number and stuff may not matter to direct customers.
After July 1, Flipkart, Ebay and Amazon may adopt the following approach (some changes depending on your state and product or type of service)
1. Register for VAT/ST.
2. Get LLP or Pvt ltd for incorporation benefits.
3. If choosing sole proprietorship, or other, choose GST option.
4. If turn over is above 10L then service tax registration is must.
Instamojo, Paytm is already asking for the GST PIN or Number starting from this month.