Shopify allows you to add a custom domain to your store. To add a custom domain, do the following: Go to Settings -> Domains and add a primary domain.
Sometimes - soon after you add a primary domain - it may show you an error when you try accessing the store via https. However, you will be able to access the store via http - if http is not forwarding http to https. This mostly happens because of the following reasons:
It is a delayed DNS propagation and it should start working properly in 24-48 hours.
It already is working, but you are seeing the cached version.
Start by clearing your cache and your cookies, or visit your website on another browser or in incognito mode. If you are able to now see the website load without any errors in http and https - it most likely was just a stored cookies issue.
Despite your best SEO efforts, you could still be missing out on another source of organic traffic: your website's images. Follow this link to know more.