Firstly, you can buy ssl anywhere, you can have your host install it for you.
To turn it on you must edit the config files located at
includes/configure.php
edit this line
Code:
// Use secure webserver for checkout procedure?
define('ENABLE_SSL', 'false');
make false to true
admin/includes/configure.php
edit this line
Code:
// secure webserver for catalog module and/or admin areas?
define('ENABLE_SSL_CATALOG', 'false');
define('ENABLE_SSL_ADMIN', 'false');
make false to true
Or you can follow https://www.zen-cart.com/tutorials/index.php?article=15