If your server has a support page, you should report any bugs/issues you encounter there first. Reporting to your support page before reporting to the developers makes their job easier, as they don't have to deal with bug reports that might not have anything to do with them. This helps us get new features faster.
If you're a technical user, or your site doesn't have a support page, you'll need to use the Bug Tracker. Please perform a search to see if there's already an open bug that matches yours before submitting anything.
Try to provide as much information as you can about the bug, including the full text of any error messages or notices, and any steps required to replicate the problem in as much detail as possible. It's generally better to provide too much information than not enough.
See this article to learn more about submitting good bug reports.
And last but not least: Better report an issue you encountered even if you don't write the perfect bug report!