Record the purchases of goods on account in a three column purchases journal?

for every question it reads similar to this: jan 3. Purchased dresses for $1250.00 plus a frieght chage of 25 plu GST from World of fashions, Invoice 101, dated December 25: net due and payable in 30 days. i put the 1250 in the merchandise inventory colemn now how do i get the gst amount and then i total them together and put my answer in the accounts payable? please help..