There are a TON of CMS's out there to choose from, but a good place to start would be to write down what you want your site to accomplish first. if you need a basic site that can easily be edited from a WYSIWYG editor, you'll have a bunch to choose from. If you're looking to build a more complex site that needs to take into consideration user ACL, social aspects, banner ads, subscriptions, eCommerce, ect, then you'd starting to narrow down the prospects.
Another thing to consider might be what language you're more familiar with (PHP, ASP, ASP.NET) since there are CMS available in all of them.
For me, I prefer using as much Open Source software as I can which made me eliminate ASP.NET Storefront, DotNuke, and some others. After that i looked at some of the more popular systems out there and looked to see how many people were using it, what they were using it for, how many people/groups were actively developing extensions for it, etc. I ended up going with Joomla which has allowed me to easily design different "themes," add social components, eCommerce, manage site advertisers and a whole lot more. There is a HUGE community for joomla and millions of users across the globe that are actively developing "extensions" (many of which are free to use on your own sites) and answering questions on the Joomla Forums.