where do I enter cost of goods sold

Information about your cost of goods sold for your Schedule C is entered in the section that asks about Inventory for your business.

You will be asked about starting and ending inventory, which can be zero for both amounts if you don't specifically keep an inventory of goods.  There will also be a place to report the cost of inventory items that you purchased or materials that you used to produce your inventory.  Those amounts will determine your cost of goods sold.  Be sure that you do not duplicate the reporting of the cost of supplies to produce your inventory in the general expenses section under 'supplies'.  

Take a look at the following TurboTax help article for more information:

Do I need to report inventory?