Try reading this: http://docs.joomla.org/Beginners
Also make sure your hosting supports PHP and a MySQL database.
As far as extensions – read the reviews. Extensions are usually easy to uninstall if you encounter a problem and most extensions have forums pages where questions / problems are usually answered quickly.
I have many websites in Joomla and also have written a couple of modules. I recommend trying Joomla out – you can easily build some brilliant websites.
on: 23rd October 10