Read the FAQ
Read the documentation
Some documentation is available on this wiki.
Search Before Posting
There is a good chance you're not the first person to experience the issue you are having. By searching the forums, you may be able to find posts about the problem you are experiencing. Also try searching the sf.net forums and the support, patches, bugs and feature request trackers on the sourceforge.net project page, and make sure you're not creating a new topic on someting that has already been discussed before (known as posting a duplicate or 'dupe').
Post in the Approriate Place
There are different places to post different things, and we ask you to only post your issue in one place.
First of all, feature requests should go in the Feature Request Tracker on sf.net. Similarly bug reports should go to the Bugs Tracker and patches to the Patches Tracker. Are you looking for support, you can either post in the Client Support Forum for FileZilla client specific questions, the Support Forum for issues with FileZilla Server or in the Support Request Tracker on the sourceforge.net project page. You can also try the IRC channel #FileZilla on irc.freenode.net.
Be Patient and Have Good Manners
Remember that support provided for the FileZilla project is made by volunteers; no one is paid to answer you. So, please, do not demand an answer, ask for one. Using good manners and words like "please" and "thank you" go further than you might think.