Before reporting any potential issues first make sure you are using the current version of the specific software in which you are experiencing the issue. You may also want to join the NIX Discord chat and describe your specific issue in the appropriate support channel in order to help identify if the problem is user/configuration/other error or an actual bug. Doing so may also alert you to any known workarounds you might be able to use in the meantime.
If you are sure that your issue is indeed a bug, your next step should be the corresponding NIXPlatform GitHub repository, ie: NixCore, Nix-GUI, nixwallet-spv-android, etc. Once there, click on the issues tab near the top of the page and check to see if there is an open issue already describing your problem.
If none exists you will need a GitHub account in order to create a new issue. This will ensure that the development team is made aware of your bug report. Accounts are free and easy to setup.
When opening a new issue, try your best to be descriptive and provide any relevant log file excerpts that can help detail your problem. The most likely source for this information will be found in the debug.log file in your NIX data directory.
Developers may ask followup questions, so be sure to reply or your issue may be closed due to the lack of additional requested information.