Well as monaghan said, its probably best for you to go with a content management system. You can find them on many script sites such as resourceindex.com, or hotscripts.com. Although opensourcecms.com is a good starting point.
Although I agree with monaghan, I think using a CMS like joomla might be too complex for what your looking for. I have had first hand experiences from many CMS systems and I would recommend either:
EcwCMS:
Website:
www.ecwcms.com
Requiements: MySQL, PHP5.
This is a good CMS that is very simple to administer and incorperate with your templates, the only downside is that PHP5 is required and not many website hosts have upgraded due to the problems associated with it. You can try EcwCMS at opensourcecms.com.
Another good CMS I have used is called MiniCWB. Its is small, decent CMS for simple editing and creation of webpages. Its very customiseable and can be found here:
www.mini-open-cms.com. The good points about this particular CMS is its very lightweight and it does not require a MySQL database, and it runs happily on either PHP4 or 5.
Hopefully that helps,
Pho~