I have a host, but what do I need to do to make a portion of the site secure.
You need a certificate for HTTPS.
DO you want that or do you want to just PW protect a directory?
if you want to protect certain parts of your site, then password protects certain parts of you site, which you can do it simply using your control panel.
Please, note that securing a server and https is two very different thing.
Https is a method to encrypt information in transit and in no way does it apply to securing a server! By default TCP port would 443 while unsecure would be 80. This SSL certificates can be created via linux using such tools as openSSL If I am not mistaken its free.
Password protect, do you mean to restrict certain portion of your site and using a password as a form of authentication? If you so, you always use .htaccess to configure that kind of stuff.
Now, securing a server. I mean physically securing your server say you have your own dedicated box. There are lot of factors to take into consideration. Your OS, what level of security you want and such.
It could be as simple as blocking certain ports on your server to a more complex task such as to lock down PHP and use mod_security with apache or securing your TMP partitions.