Any cpanel based hosting provider now provides free SSLs so just ask your provider to enable the SSL and if you are on a dedicated server where you have full root access then you just need to install SSL plugin if its not installed or if its installed just enable it.
You can check here https://www.sslshopper.com/sslchecker.html
if your hosting company has provided you auto SSL because nowadays free DV SSL certificates are provided by the hosting providers which renew automatically in 3 months.
If your web hosting provider do not offer free SSL certificate, you can register it with Cloudflare. They offer SSL free of Cost.
1) You can Setup Cloudflare Free SSL Certificate for WordPress by registering your account with Cloudflare
2) Installing free CloudFlare Flexible SSL Plugin.
My website SSL is also registered with Cloudflare.
as per above comments, looks like it due to mixed content
Install "Really simple SSL" plugin. that could help you on http redirect to https & It can too change all http request to https
Plugin link: https://wordpress.org/plugins/really-simple-ssl
If you're on a cPanel platform, then just ask your hosting provider to enable SSL certificates to all your hosted domains under your account, it is just one click. You can then manage SSL's for domains in your cPanel -> Security -> SSL / TLS Status