Https just means "http with ssl". In order to enable SSL, we need a certificate which is either self-signed or signed by an external Cerificate Authority. A self-signed certificate need to be imported when browsing. For easy-to-use purposes, use an external cerificate here. http://www.startssl.com/ and https://buy.wosign.com/ are two sites for buying cerificate. I use https://buy.wosign.com/ here.

For e-commerce websites, use OV(Organization Verified) SSL. For financial websites, use EV(Extended Verified) SSL.

Enable in Nginx

server {
    listen 443 ssl;

    server_name example.com;    
    ssl_certificate /etc/ssl/private/example.com_bundle.crt;
    ssl_certificate_key /etc/ssl/private/example.com.key;

    ...
}