Few days back i have suffered a lot while adding “www” ( world wide web ) to my domain and find many users facing this diffculties .It is quiet hard to add “www” in wordpress Url for those who are new to wordpress . 
Adding and removing “www” from wordpress Url effects yours presense in Google search so its better to set a prefered domain for search engine through Google webmaster tools ,which is possible after moving to Webmaster Tools tab and then setting a Prefered domain. 
However there are many Guides and redirect plugins that tells you to add “www ” via editing .htacess file and also some other wordpress files but some times editing this file will not help you in adding www to yours wordpress Url .
Therefore i have tested one easyiest method to add www to wordpress Url ,and this is done directly through dashboard .Remember to edit yours, .htacess File also otherwise search engine consider www and non www Url as two different Url causing duplicate content problem.
Now to add www to yours domain login to yours wordpress ->from dashboard go to -> setting and there add www before yours domain name in either both WordPress address (URL) and Blog address (URL) or according to yours preference . 
