Kindly, may I ask, if your website is actually using Cloudflare first?
If so, check that the needed DNS records are 
cloud (proxied via Cloudflare).
Furthermore, as already asked, maybe it is due to the URL configuration of your web application (which does not do auto-redirect from HTTP to HTTPS due to being working on HTTP only until now) or depending on the Web server you are using (Apache - htaccess, Nginx - vhost/conf file).
If using Cloudflare, there is an option Always use HTTPS.
Moreover, if interested into redirecting some specific HTTP to HTTPS URLs using Cloudflare, this can be done using Page Rules.
If you need option to fix your mixed content, in Cloudflare there is Automatic HTTPS Rewrites option to help you with that too.