I often find bugs in QGIS and its Plugins.
I would like to ask about this on the internet but I do not seem to find the proper place to address this issue.
Are there any instructions?
For QGIS, see http://hub.qgis.org/projects/quantum-gis/issues
For plugins, see http://hub.qgis.org/projects/qgis-user-plugins/issues
You need an OSGeo account to fill tickets. After creating that, go to your OSGeo user page. Then you can login to hub.qgis.org with the same user ID.
Many QGIS Plugins uses GitHub as their primary issue tracker. Issue tracker is usually specified in the plugin's metadata