You can find the proxy address when you visit some sites that tell you your online IP (it's your proxy's IP only when you really have a proxy):
http://btfaq.com/natcheck.pl
http://www.whatsmyip.org/
http://www.all-nettools.com/toolbox
The last site also has a proxy test. The port for transparent proxies is port 80.
In the advanced section of the proxy settings you can specify different proxies for different protocols like http, ftp, etc. If you want to use the proxy only for http communications, you have to clear out the other settings and you have to uncheck "use the same server for all protocols", of course.