You can place this rule in your DOCUMENT_ROOT/.htaccess
file:
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^mydomain\.pl$ [NC]
RewriteRule ^ http://www.%{HTTP_HOST}%{REQUEST_URI} [L,NE,R=301]
RewriteCond %{HTTP_HOST} ^(www\.)?mydomain\.pl$ [NC]
RewriteRule ^$ http://www.mydomain.pl/start [L,R]