You could create a simple one yourself, but in my opinion check out Open Source CMS Demos & Information - opensourceCMS - they have a range of demos for content management systems, you might find something you like.

If you do decide to go down the "create your own" route, you mention that only you and a few friends will be able to content, so you'd be looking at creating a simple user system to differentiate between guests and members of staff. Do you want more power over the others? Then you need to look into creating user levels / groups e.t.c.

Security. You need to make sure that any input you accept on the site (username, password, news content, any uploads) are what they say they are, and come from your site, not anywhere else. You don't want to leave yourself open to CRSF, XSS, SQL Injection attacks and so on.

I'm not sure how "simple" you need it to be. At the moment I am learning to use a php framework called CodeIgniter. I like it quite a bit. It uses MVC (Model View Controller) approach to php coding. It is totally free and has an absolute BUCKETLOAD of support documentation and a decent following I believe. Hope this helps.

CodeIgniter is not for beginners. I think Joomla is the most easy one you can use for start.

have you considered Wordpress? it has CMS capability and is written in PHP. there are many references that you can look up, both online and offline that provide guidance on code tweaks.

Concrete5 is another options. Easy to use in page editing you dont have to sign in etc.

concrete5 - Free CMS | Open Source Content Management System

use wordpress, is verry easy and light

Well, If you are not an advanced user with PHP, Codeigniter would
not be for you, but if you are - THAT'S EVEN BETTER! Use a pre-created
CMS for your use.