Do you mean page title tag for Joomla?
This is not as easy as one would imagine and I plan on writing a very detailed SEO guide for Joomla shortly.
First check your global configuration and look for the meta setting - make sure "Show Title Meta Tag" is on.
I would recommend you investigate a number of plugins and grab one to help you, and Joomla, with this.
sh404sef is brilliant and I recommend this first. It will let you do title tags for all your pages but may take a while to get used to - you'll be going back and adding page title tags by search engine friendly URL - you wont be entering title tags for each content item so it will not seem as intuitive to begin with - it is the most powerful option.
Joomseo is an excellent plugin for fast titles - will simply turn your content title into the page title tag.
www.joomlatwork.com/ do an excellent search engine friendly patch which lets you add title tags for menus and content items - very good component but I found it did not do titles for some components such as virtuemart. It's more intuitive than sh404sef, and possibly has some other benefits as well, but sh404sef is more powerful for handling meta because you can address every URL in your site if you have the patience ;-)
