So you want your website to use HTTPS. You want to secure your website and enjoy the search engine benefits. In order to get that sweet padlock, there are a couple of things you need to do.
Before your website can use HTTPS, you need to have an SSL certificate for your domain name. You will need to get this SSL certificate from an SSL certificate vendor and install it on your hosting account.
You can install your certificate using the SSL/TLS section in your control panel. For more information, see this article.
After installing an SSL certificate, you need to make sure your visitors are actually using https. To do so, you need to redirect all traffic to the https version of your website. See this article for a few ways to do that.
If your website has a valid SSL certificate and is using https, but you do not see a padlock, this is because some of the URLs on your website do not use https.
The next step is to find out what generated the bad URLs and how to fix them. Unfortunately, there is no general method to do so: it depends on many aspects of how you built your website.