QGIS has a page Bug/Issues Page.
QGIS is a largely volunteer driven project, and is the work of a dedicated team of developers, documenters, translators and supporters. Despite the efforts of the team to release QGIS without bugs, there may remain some bugs. If you find a bug or want new features to be added, please report it!
Plugin bugs must be opened in their respective bug tracking system. Check first if the plugin is listed in the plugin overview. If so, click on the plugin name then click “New issue”.
Most of the plugins are published in the official QGIS Plugins repository. If the plugin is available, you’ll find in its presentation a link to its bug tracker. Otherwise, consult the plugin documentation to find the address of the relevant bug tracking system or a developer to contact.
I would also keep in mind:
Before filing an issue, review the currently open issues to make sure that you aren’t creating a duplicate. If you have additional information on an issue, you can add it to the existing ticket. Third party plugins might also cause problems. If you have installed any, you should also verify that the problem is still reproducible without them. Please don’t report multiple unrelated bugs in a single bug report.
In Linux, you can create a backtrace
Creating a backtrace
If you have a crash it might be useful to include a backtrace as the bug might be not reproducible on another machine.
On Linux QGIS automatically tries to use gdb to connect to the crashing process to produce a backtrace. But some distributions disable the possiblity to connect debuggers to a running processes.
Log output on Windows
The nightly build in OSGeo4W (package qgis-dev) is built with debugging output, that you can view with DebugView. If the problem is not easy to reproduce the output might shed some light about where QGIS crashes.