If your annual turnover is less than and equal to 1.5 cr than you can opt for quarterly return (GSTR-1).
If you are the regular dealer than you have to required file two returns each month
1) GSTR-1: GSTR-1 file furnished with the details of outward sales transactions, debit and credit notes of a particular month.
2) GSTR-3B: Payment of tax liability(tax collected in case of sale and other incomes which is under GST purview) and availing input tax credit(tax paid while purchasing and for incurring direct/indirect expenses).
The amount you have to pay is:-
= TAX LIABILITY – ITC(Input tax credit)
= (₹6,00,000 x 12%) – (₹7,00,000 x 12%)
= ₹72,000 – ₹84,000
In your case, the purchase is more than sale so you have extra input available after offsetting the tax liability. so you didn’t need to pay any.
Hope this helps. Have a good day!