After you Magento is installed you will need to set up your website, your Magento store. This often has the default details filled like Title,Meta Description and keywords.
You need to edit these to optimize for SEO.
These are the simple steps to take in order to change these:
Step 1 – login to your Magento backend usually is www.yourdomainname.com/index.php/admin or if is customized to something different admin123 or any other custom extension
Step 2 – Go to System -> Configuration
Step 3 – On the left menu, choose Design
Step 4 – Click on the dropdown called HTML head, under this section you can edit the Default title, meta description and keywords.