Wordpress, drupal, joomla, the list goes on and on. They all have their pros and cons but all can get the job done for you. My advice is to find one that you like the look and feel of.
And also, more important than anything else with open source CMS apps, set up a weekly backup plan, make sure you dump your mysql database to a text file once a week and back that up as well. And either subscribe to the CMS site so you get emails of the updates or monitor them closely.
Since these are free and open source the code is available to everyone and these applications are by far one of the most vulnerable to compromises if you do not keep up to date, on the opposite side of things the compromises are usually fixed very quickly after being discovered and an update is put out, so make sure you stay up to date.
I couldnt even tell you how many people I spoke to where some 12 year old kid found a script on google and compromised their site because they were out of date and because they had no backup plan had to basically start their site from scratch. Don't let the 12 year old script-kiddie win...