Whether what amount i have to pay tax

I m new register person opted for composition scheme , i have small shop of garment all earlier stock is purchased without bill donot have any record of stock, all other are saying first i have to paid the Tax on earlier old stock, then have to filed composition scheme return.

Is this true, if yes what amount of Tax i have to be paid , when have to be paid and where have to be paid.And though i have no record of my stock how can i calculated my stock.

If you kindly clear my confusion i will be very much grateful to you.

Thanking You