How to apply a content management system to a website made in Dreamweaver?

Hello,

I've recently put together a simple website (designed in photoshop/ sliced/ dreamweaver) for a local charity, but am unsure how to help them update their site and upload pictures themselves.

I've heard of content management systems but have no idea how they work, which one to go for or how to apply them to my designs. Does it have to be designed in a certain way, or built in to the original?

Any help would be much appreciated :)

xxx