Please Help me in htaccess file mod_rewrite + redirect

This is my .htaccess file and working fine.
What I want If any one try to browse the site without www , this have the capability to redirect the url with www.

Example:- if someone try to browse with
http://mydomain.com/theater/city/123
then this will be redirect to
http://www.mydomain.com/theater/city/123

My current .htaccess file below.

Options +FollowSymLinks
Options -MultiViews
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteRule \.(gif|jpg)$ - [F]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d


RewriteRule ^city/(.*)/(.*)$  index.php?city=$1&page=$2 
RewriteCond %{QUERY_STRING} !page
RewriteRule ^city/(.*)$       index.php?city=$1 [L]



RewriteRule ^cinemas/(.*)/(.*)/(.*)$    theater.php?thid=$3&cityname=$1 [L] 
RewriteRule ^cinemas-new/(.*)/(.*)/(.*)$    theater-new.php?thid=$3&cityname=$1 [L]	
RewriteRule ^sitemap.xml(.*)$           sitemap.php [L]  

Code (markup):

Thanks in advance I really require this.

 

# force www. start
RewriteCond %{HTTP_HOST} !^www\.yourdomain\.com [NC]
RewriteRule ^(.*)$ http://www.yourdomain.com/$1 [R=301,L] 
# force www. stop

Code (markup):

 

Here it goes

Options +FollowSymLinks
Options -MultiViews
RewriteEngine on
RewriteCond %{HTTP_HOST} ^domain.com [NC]
RewriteRule ^(.*)$ http://www.domain.com/$1 [L,R=301]
RewriteCond %{HTTP_REFERER} !^$
RewriteRule \.(gif|jpg)$ - [F]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d


RewriteRule ^city/(.*)/(.*)$  index.php?city=$1&page=$2 
RewriteCond %{QUERY_STRING} !page
RewriteRule ^city/(.*)$       index.php?city=$1 [L]



RewriteRule ^cinemas/(.*)/(.*)/(.*)$    theater.php?thid=$3&cityname=$1 [L] 
RewriteRule ^cinemas-new/(.*)/(.*)/(.*)$    theater-new.php?thid=$3&cityname=$1 [L]	
RewriteRule ^sitemap.xml(.*)$           sitemap.php [L]
 

Code (markup):

 

Hello All,

I am really thanks to u all, otherwise I have to do struggle a lot to achieve the same.