Thanks again for your quick reply.
The point is, I don't have ISPconfig installed. I haven't even forwarded the HTTP port in my router settings, because I only access the server on my LAN. I want to use it as a "playground" to get familiar with Linux and develop my website, so that I can start right away when I'm renting a web server.
So the only document root I have is /var/www/. I have put there every file I need (php files, html files) and I can access them perfectly. Yet, .htaccess does not work.
I hope the problem gets clearer now