If you have never set your preferred domain to either with www or without www, then it might cause problems with search engine indexation, search engines might split your backlink count. The following methods showed you how to redirect a web page:-

1) Set a 301 Redirect

The best way to solve this problem is to set a 301 redirect, but I have never tried before as I have little confused about the coding of 301 redirect.

2) Install Enforce www. Preference Plug-in

Install and active the Enforce www. Preference plug-in, it will redirect your URL to http://www.domain.com when we enter http://domain.com.

3) Use Google Sitemap to Set Preferred Domain

Login Google Sitemap, under tools > set preferred domain. There are 3 options for you to choose as below, select one of them as you wish.

Option 1: Display URLs as www.blogging-secret.com (for both www.blogging-secret.com and blogging-secret.com)
Option 2: Display URLs as blogging-secret.com (for both www.blogging-secret.com and blogging-secret.com)
Option 3: Don’t set an association.

4) Configure Through Wordpress 2.3 and above

I’m using this method now. Login your Wordpress (http://www.domain.com/wp-admin), click on “Option” tab, and then choose “General” option, you will see Wordpress address (URL) and Blog Address (URL).

Just change it to http://www.domain.com if you want your domain name appear http://www.domain.com when we enter http://domain.com, vice-versa.