The two returns you deal with most
GSTR-1 reports outward sales. GSTR-3B is a summary return where you pay net GST.
GSTR-1
Lists all sales invoices, split into B2B and B2C, plus an HSN summary.
GSTR-3B
Output tax (on sales) minus input tax credit (on purchases) = net tax payable.
Prepare easily
Software that records every invoice can export a ready GSTR-1 and show a GSTR-3B summary.