All your sites should have valid SSL certificates installed already. If your site is showing as not secure, check to make sure that all components on a page are using https otherwise the page will show insecure. Firefox is the best thing to use to check.
Go to: https://yourdomain.com/
in firefox. If the lock symbol next to the url in the browser shows a warning, right click the page and click View Page Info, then click the Media tab.
This will list the url's that the images are using. Some may be using https already, but, some may be hard coded with http causing the page to show insecure:
are two examples. The html probably looks something like:
so the url is 'hard coded'. If you changed that to:
Then the browser will automatically fill in the first bit depending on what the visitor put in their browser. IE: