For each of your products, you can add options that affect the price of the product. For example, if you sell a shirt in size small and large, you can have one product listing but charge $10 for the small and $12 for the large.
If you haven't set up any products yet, follow the step by step instructions in the Add Products article. The options setup is in the 'Product Variants' section.
If you already have your products listed, follow these steps:
Go to your catalog page and click on the product you would like to add options to, then click 'Edit Page.'
Scroll down to 'Product Variants' and checkmark the box. Below that some options will appear.
Enter a 'Variant Title' like Size or Color. Click 'Add Variant.'
Fill in the product information. Note that '# in Stock' only appears if you have checkmarked 'enable inventory management for this product.' Then, click 'Add Variant.'
Your option appears in the list and you can now add more variants if you like. If you would like to save time by copying the information in one of your variants, click the 'Copy' link to the right of any variant.