To add a new product to your online store, please, go through the following steps:
Add new product clicking on one of 'Add product' buttons in the upper right corner or in the center of the page(Fig. 3.1 - [ 3 ]).
Also, you can import products to your store using CSV file (Fig. 3.1 - [ 4 ]). More about import is in Shopify Manual
Very explicit instructions how to work with product in Shopify theme you can find in this Shopify help guide.