I can't get too detailed as to what's usually wrong, because it's generally part of the assignments to get this right -- but go back to examples you have been presented with, and make sure your CGI is sending back everything it should, and in the order is should.
You can generally get a better error message by looking at the server error log, which is /var/www/logs/error_log. Make sure you look at the log for the right machine -- remus or romulus. A simple grep for your username through this file will generally show you (all :-) the errors you're committing.