The most important thing is before reporting a bug, try to find out if it just isn't some misconfiguration that you did yourself.
Like you did by asking here or you could ask in mailing lists too ( http://lists.debian.org
Then you use reportbug to report it.
Here, it's important to check if the bug wasn't reported already.
Unfortunately, some maintainers also almost never close bugs so the list for some packages are filled with duplicates, already fixed bugs etc. But that's a problem for them to deal with, you just do your best.
Reportbug's graphical interface used to crash quite often (maybe the wheezy version is better) but you can use the text interface which doesn't fail (it's not complicated).
If you have a gmail account, set it up in reportbug like it's said in the wiki as it works better than the default server (I think other webmail servers could be used similarly).
Last, after the bug is reported, you receive a confirmation in your email. You won't be automatically subscribed to the report (so if someone posts something and doesn't include your email address you won't see). To subscribe to it, just click on the link of the bug page you will receive and then at the bug page click on 'subscribe'.