Bug reporting and feature requests are done using GNOME's Bugzilla interface. You will need a bugzilla account to file bugs and comment on them. See also the Example queries section at the end of this page for some useful links for searching PiTiVi's bug list.
Remember to take a look at the existing list of bugs/feature requests to see if your problem has already been reported (hint: use the control+F functionality of your browser!).
If you need to report a crash bug or incorrect behavior, try to provide the debug log when possible. A simple debug log can be obtained by running pitivi with this command:
PITIVI_DEBUG=*:5 GST_DEBUG=2 bin/pitivi > debug.log 2>&1
The resulting file will probably be too big to be attached to a bug report directly. Instead, try compressing it (in gzip, bzip2 or lzma format) before uploading it. If you want to add debugging information from GNonlin, use a command like this:
GST_DEBUG=3,gnl*:5,python:5 PITIVI_DEBUG=5 bin/pitivi > debug.log 2>&1
When reporting a crash, it would be good to provide a stack trace. See Getting Traces for instructions.