1. Create new file with you favorite text editor and save it as: .htaccess
2. Copy/paste these lines in your file:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^domain.com$
RewriteRule (.*) http://www.domain.com/$1 [R=301,L]

3. Replace “domain” with your domain name

For example, rewrite for my domain (http://www.bobz.com.hr) would be:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^bobz.com.hr$
RewriteRule (.*) http://www.bobz.com.hr/$1 [R=301,L]

4. Save file and upload it to your web site root directory (public_html)

Also you can revers this, if you want to display your domain as: http://domain.com, then you would use this:

RewriteEngine on
RewriteCond %{http_host} ^www.bobz.com.hr$
RewriteRule ^(.*)$ http://bobz.com.hr/$1 [R=301,L]

I think all of modern CMS systems have url rewrite by default included, so you can use built in functions. This example is useful if you are working on a static HTML site.
Be sure that you are using Apache web server and have enabled mod_rewrite module before using these rules.