How to use HTTPS on your webstore
The use of HTTPS on your webstore is an absolute must for attaining the trust of your customers and keeping their data encrypted.
HTTPS on Subdomains
If you are using a subdomain on your webstore then you can enable the HTTPS redirect on your store from the "Settings > Domain name" page and this will make any connection to your webstore default to a HTTPS connection.
HTTPS on Custom domains
If you want to make sure that all of your traffic is using HTTPS when you are using a custom domain then you will need to purchase an SSL certificate for your custom domain. After you purchase it can take up to 15 minutes to work correctly on your webstore and the HTTPS redirect will automatically be turned on for your webstore.
If you find that after redirecting to HTTPS on your webstore does not give the green secure notification in front of the domain in the URL then the main cause of this would be insecure links or assets on your webstore.
You can often find the cause in background images linked to with "http" in your theme or template, though it could include assets in your descriptions or CMS pages as well. If you want to pinpoint the problems you can find them using Chrome's "Inspect" function under "Console".