Create .htaccess file in the document root directory i.e. the directory www.mydomain.com points to. Put the rewrite directives in .htaccess file. Use this method when you do not have access to the httpd.conf file.

Most web hosting servers have the mod_rewrite installed and running for this stituation. If not ask then to install it and set mod_rewrite directives in the .htaccess file.