QGIS is under active development and as such it won't always work like you expect it to. The preferred way to get help is by joining the qgis-users mailing list. Your questions will reach a broader audience and answers will benefit others.
This mailing list is used for discussion about QGIS in general, as well as specific questions regarding its installation and use. You can subscribe to the qgis-users mailing list by visiting the following URL: https://lists.osgeo.org/mailman/listinfo/qgis-user
If you are a developer facing problems of a more technical nature, you may want to join the qgis-developer mailing list. This list is also a place where people can chime in and collect and discuss QGIS related UX (User Experience) / usability issues. It's here: https://lists.osgeo.org/mailman/listinfo/qgis-developer
This list deals with topics like documentation, context help, user guide, web sites, blog, mailing lists, forums, and translation efforts. If you would like to work on the user guide as well, this list is a good starting point to ask your questions. You can subscribe to this list at: https://lists.osgeo.org/mailman/listinfo/qgis-community-team
This list deals with the translation efforts. If you like to work on the translation of the website, manuals or the graphical user interface (GUI), this list is a good starting point to ask your questions. You can subscribe to this list at: https://lists.osgeo.org/mailman/listinfo/qgis-tr
This list is used to discuss Steering Committee issues related to overall management and direction of QGIS. You can subscribe to this list at: https://lists.osgeo.org/mailman/listinfo/qgis-psc
In order to locally promote QGIS and contribute to its development, some QGIS communities are organized into QGIS User Groups. These groups are places to discuss local topics, organize regional or national user meetings, organize sponsoring of features... The list of current user groups is available at https://qgis.org/en/site/forusers/usergroups.html
You are welcome to subscribe to any of the lists. Please remember to contribute to the list by answering questions and sharing your experiences.
We also maintain a presence on IRC - visit us by joining the #qgis channel on irc.freenode.net. Please wait for a response to your question, as many folks on the channel are doing other things and it may take a while for them to notice your question. If you missed a discussion on IRC, not a problem! We log all discussion, so you can easily catch up. Just go to http://irclogs.geoapt.com/qgis/ and read the IRC-logs.
Commercial support for QGIS is also available. Check the website https://qgis.org/en/site/forusers/commercial_support.html for more information.
While the qgis-users mailing list is useful for general 'How do I do XYZ in QGIS?'-type questions, you may wish to notify us about bugs in QGIS. You can submit bug reports using the QGIS bug tracker.
Please bear in mind that your bug may not always enjoy the priority you might hope for (depending on its severity). Some bugs may require significant developer effort to remedy, and the manpower is not always available for this.
Feature requests can be submitted as well using the same ticket
system as for bugs.
Please make sure to select the type
If you have found a bug and fixed it yourself, you can submit a Pull Request on the Github QGIS Project.
The QGIS community also runs a weblog at https://plugins.qgis.org/planet/, which has some interesting articles for users and developers. Many other QGIS blogs exist, and you are invited to contribute with your own QGIS blog!
The website https://plugins.qgis.org is the official QGIS plugins web portal. Here, you find a list of all stable and experimental QGIS plugins available via the 'Official QGIS Plugin Repository'.
Lastly, we maintain a WIKI web site at https://github.com/qgis/QGIS/wiki where you can find a variety of useful information relating to QGIS development, release plans, links to download sites, message-translation hints and more. Check it out, there are some goodies inside!