I'm not a finance person, what books/records do I actually need to keep from day one?

At minimum: a sales register (every order, with GST split if applicable), a purchase/expense register (raw materials, ads, shipping, software), a bank reconciliation, and inventory records (what you bought vs what you've sold vs what's left). Almost every founder can skip spreadsheets and go straight to a proper accounting tool (Zoho Books, Vyapar, Tally) from month one, GST invoicing and return prep alone make it worth the Rs 500-1,500/month. The real discipline isn't the tool, it's doing it weekly, not saving three months of receipts for your CA at year-end.