Easy Frontend SEO is a very nice extension from Kubrik extensions. They also have some excellent other extensions, like the SIGE Gallery, a Guestbook (Easybook Reloaded), plus another little SEO plugin for optimizing links and images (SEOFLI). The extension is made to very easily manage your meta-data. You can do so from the front-end of the page you are actually looking at, so no need to doubt whether you should create meta-data for an article, or the menu-item, or even somewhere else.
To use the extension, download the extension, install it through the Extension Manager and enable it through the Plugin manager:
If you are signed in on the front-end of your site, you will now see a little set of icons at the top of the screen:
Based on the default parameters, this tells us that there is a title, no meta-description, no meta-keywords, and there is a robots tag and a generator tag (mouse-over tells you what is what, configurable). So if you click one of the red crosses, it allows you to set your meta-data. In an unconfigured site, they will be empty, so just fill them in as desired. The default settings also stop you from entering more then the advised number of characters for meta-tile (60) and metadescription (160). Also this can be overwritten in the configuration.
If you now go back to the plugin manager's setting for this plugin, you can set the Automatic mode. This will create meta-data based on the content of your articles. Look what happens when we activate the feature and check one of the articles from the joomla example content:
Though not perfect, it does a rather good job, and at least something has been filled in. As a first step you could activate this feature, and then go over the separate articles to fine-tune the meta-data.
The extension is available for free for both Joomla 2.5 and 3, though a donation for the developer would be nice.