The adress of our bug tracker is:
There you can report bugs, submit feature requests, patches, or request to merge your branch if you developed something using git. The tracker is divided into modules, so please be specific and file your request in the appropriate subsection. In case of doubt, leave it in the "FreeCAD" section.
If you think you might have found a bug, you are welcome to report it there so long as you have first discussed the matter in the appropriate forum. But before reporting a bug, please check the following items:
If you want something to appear in FreeCAD that is not implemented yet, it is not a bug but a feature request. You can also submit it on the same tracker (file it as feature request instead of bug), but keep in mind there are no guarantees that your wish will be fulfilled.
In case you have programmed a bug fix, an extension or something else that can be of public use in FreeCAD, create a patch using the Git diff tool and submit it on the same tracker (file it as patch).
If you have created a git branch containing changes that you would like to see merged into the FreeCAD code, you can ask there to have your branch reviewed and merged if the FreeCAD developers are OK with it. You must first publish your branch to a public git repository (github,bitbucket, sourceforge...) and then give the URL of your branch in your merge request.