How can I do redirect no www to www?

Put in the following code in different places in htaccess but redirection is not working.
RewriteCond %{http_host} ^tooba.me [nc]
RewriteRule ^(.*)$ http://www.tooba.me/$1 [r=301,nc]

Please, help me.

This is no Zenphoto issue. Please use a web search, for example: http://www.htaccesstools.com/