How to redirect non-www site to www site?

[admin]

I have created one wordpress site.
As I write domain name without www then it opens correctaly
but as I write www. in url it's not showing the site.

Please help me..
Thank you in advance.

I have edited my question and added the following part :

Following is my file :
Where I have to put code provided by you ?
I tried it before the "Begin Wordpress" line but still not working.

.htaccess

"# -FrontPage-"

IndexIgnore .htaccess <em>/.??</em> *~ *# <em>/HEADER</em> */README* */_vti*



order deny,allow

deny from all

allow from all





order deny,allow

deny from all



AuthName example.com

AuthUserFile /home/example/public_html/_vti_pvt/service.pwd

AuthGroupFile /home/example/public_html/_vti_pvt/service.grp

"# BEGIN WordPress"



RewriteEngine On

RewriteBase /

RewriteRule ^index.php$ - [L]

RewriteCond %{REQUEST_FILENAME} !-f

RewriteCond %{REQUEST_FILENAME} !-d

RewriteRule . /index.php [L]


"# END WordPress "