First, go on the project page and check for information on how to report bugs. They might have a preferred way of doing it.
Most projects have mailing lists. Most of them have a user and a developer mailing list. Start by searching the lists to see if the bug you have discovered was already discussed. Maybe it's not a bug and the product simply does not support what you try to do.
If you have already digged in the code and found the cause of the bug (and maybe the fix), subscriber to the developer list and post a message describing the problem. Include a complete description of the problem, the version you use (and the version of other software if needed. ie.: Web server, OS, ...), a test case, what you found in the code and the patch you made. If it's a bug, they will tell you to report it in their bug tracking software (bugzilla, mantis, redmine, track, ...)
If you don't find anything in the code, subscribe to user list and post your problem.
Avoid saying thinks like "please, I really need to fix or I ...". Open source developer are not your employees. If you want something fixed, you can always do it yourself. Avoid ultimatums and rant about the software.
If the bug was already reported, the only thing you can do is watch it or vote it up. Avoid adding comments like "me too!" or "we need this fixed!" or "why is this still not fixed?!?". That is annoying.