Cal here from Shopify, thanks for reaching out to us. Shopify themes are used to create the ideal look and design for your store! We have a range of free Shopify themes that you can install on your store, and if you're looking to upgrade down the road, we have a range of paid themes from a host of third-party theme developers.
Is your business just getting started? What were you hoping to sell? I would recommend installing and playing around with one of the free themes so you can get to grips with designing your store! You can learn more about using themes here.