You need to purchase a SSL Certificate and give your site URL to install.
Some plugins available for SSL certificate setting on your site.
1. Really Simple SSL
2. WP Force SSL
But after installation SSL on your domain, you need to some setting on your phpMyAdmin section, click on your database and go to wp-options section update site URL and home URL with HTTPS and save. Now your site is ready with https://domain-name.com.
Form more in detail go to More Info