If you' re not on [https://www.shopify.com/plus](Shopify Plus) and want SSL you need to either go through a cloud service and redirect your DNS or force SSL by checking the URL protocol with javascript and from here re-directing to https://
Please note that by forcing SSL with a javascript redirect you might run into issues and these issues might vary. Check any third parties you've got like google analytics, assets urls etc.
Read more about shopify and protocol URL's in the shopify wiki
https://docs.shopify.com/support/your-website/themes/protocol-independent-urls