I need to learn dynamic development. I know XHTML and CSS and own Dreamweaver CS3.
Should I learn PHP or ColdFusion? What is the best way to learn? If I go with CF and have the developer version, will it cost me anything?
Well it really depends on how you want to build, I don't really have a lot of experience in CF and don't really care to but one of the main things that drew me to PHP was just how much information there is out there for it, mainly due to it being open source. But don't think PHP and CF are your only choices; Python is making it's way into the web with things like Django, and then there is Ruby on Rails which I won't comment on due to lack of experience.
As mentioned previously PHP is good option as it is free to use, have huge support in community (there are many scripts/pluging to enhance usability for free), able to work with other technologies (XML, DB conectivity, JS, AJAX) and also let you develop pages for mobile devices (mobile phones, PDA, blacberry)
If I go with CF and have the developer version, will it cost me anything?
From what I can tell on the Adobe site, as is anything Adobe, it's a 30 day free trial and then you pay! $1299 for the standard edition and $7499 for the enterprise edition. Unless you plan to host the site yourself, you'll find that your host choices are going to be limited and much higher priced than if you would have went with php. It's rare to find a host that doesn't support php. Also, there's not many resources for noobs. Compare the two forums on this site. 9 pages of CF posts versus 153 of PHP. I've never used CF for these reasons.
I agree that you should go to apachefriends.org and download the installer version of XAMPP. As soon as the download is complete, follow the install instructions. After installation is complete, open the xampp folder and double click apache_start. You now have a fully configured testing server up and running on your machine. Save your php files in the htdocs(root) folder and view the files by typing http://localhost/filename.php into the url.
What do you think the best way to learn the BIG PICTURE in all of this?
Start off with a two page form. Learn how to pass variables from the client back to the server. Here's a good tutorial to start with.
Read posts, ask questions, and utilize google. You'll have dynamic content in no time!