Julie here from Shopify Support.
Great question! Your website URL is the same name you entered when you first signed up, followed by .myshopify.com. If there are any spaces in your store name, then they are replaced with hyphens. For example, if your store name is "My Store", then your URL would be "my-store.myshopify.com".
You can find out what your URL is by heading to Online store > Domains. There, you'll notice a primary domain, which is the URL that's displayed in the address bar while customers browse your online store. This is the same URL you'll want to provide customers.
That being said, most merchants choose to replace their myshopify.com URL with a custom domain, which are usually just regular .com domains. You can actually purchase domains directly through Shopify; on average, they're about $14 USD per year. Using a custom domain definitely adds a professional touch to your website and also makes your URL easier for customers to remember, since they won't need to enter the .myshopify.com portion.
I hope this helps!