If you want to write the PHP code yourself, you'll need to learn:

1) How to create HTML Forms.
2) How to have PHP handle HTML form submission.
3) How to connect to a database with PHP
4) Saving data to the database
5) Retrieving data from the database
6) Creating a Login Form and Authenticating Login requests
7) Persist HTTP Requests using PHP Sessions

The other option is to use a CMS, then you'll have to learn how to setup the CMS, and the admin interface of the CMS.

Which would you prefer?

inititially the cms, the php will follow with time.

If your site is a blog site, wordpress (http://wordpress.org/) is really good.
If you have a more general site, Joomla (http://joomla.org/) and Drupal (http://drupal.org/) are the top ones for PHP.

Heres some resources on choosing a CMS:

http://www.cmsmatrix.org/ - select a list of CMSs and cross reference features.
http://opensourcecms.com/ - Try demo's of open source CMS's.