First, make sure it's a bug. You should also search in the issue tracker and forums to see if anyone has already reported this bug.
Once you've established that it's a bug, and that we don't know about it already, the most important thing you can do is come up with a simple description and reproduction recipe. For example, if the bug, as you initially found it, involves five files over ten commits, try to make it happen with just one file and one commit. The simpler the reproduction recipe, the more likely a developer is to successfully reproduce the bug and fix it.
When you write up the reproduction recipe, don't just write a prose description of what you did to make the bug happen. Instead, give a literal transcript of the exact series of commands you ran, and their output. Use cut-and-paste to do this. If there are files involved, be sure to include the names of the files, and even their content if you think it might be relevant.
In addition to the reproduction recipe, we'll also need a complete description of the environment in which you reproduced the bug. That means:
1. nopCommerce version
2. Expected behavior
3. Actual behavior
4. Steps to reproduce the problem
5. Any private modifications you made to your nopCommerce
Start out with a clear description of what the bug is. That is, say how you expected nopCommerce to behave, and contrast that with how it actually behaved. While the bug may seem obvious to you, it may not be so obvious to someone else, so it's best to avoid a guessing game. Follow that with the environment description, and the reproduction recipe. If you also want to include speculation as to the cause, and even a patch to fix the bug, that's great.
Thanks. We know it's a lot of work to file an effective bug report, but a good report can save hours of a developer's time, and make the bug much more likely to get fixed