How do I figure out the cost of merchandise purchases for the year?

In the account book, under payables you should have everything you spent on purchases. When you add that for the fiscal period, you should have a total.

P.S. This link may help:

Why can a retailer record its purchase of merchandise as a debit to purchases within the cost of goods sold, instead of the asset inventory? | AccountingCoach.com Q&A (http://blog.accountingcoach.com/purchases-merchandise-inventory/)