Did you know that search engines index pages with and without the "www" (i.e. www.blah.com and blah.com)?  Although, they appear the same Google sees them as two separate pages.  Unfortunately, it means your diluting your link popularity and possibly getting penalized for duplicate content.  Don't throw your hands up in disgust just quite yet, luckily there's an easy solution. 

1. Create a text file called htaccess.txt.
2. Copy and paste the code below into it.
3. Replace "yoursitename.com" to your domain name.
4. Upload the file to the root directory (usually public_html or something similar)
5. Test.  In the address bar of your favorite browser type in yoursitename.com and make sure it redirects to http://www.yoursitename.com/.