Karlie here from the Shopify Support Team! If you're looking to add a theme to your Shopify storefront then what you can do is first head into the Shopify Theme Store, here. Once you've found a theme that would work for your online business, you can add that theme to your Admin.
Next, you will want to publish your theme to have the theme show up for your customers on your online storefront. To publish a theme:
I hope this has answered your question. If not, or if you have any further questions please let me know. I will be happy to help!