You can check the Slicer 3D Documentation, maybe starting from here: https://slicer.readthedocs.io/en/latest/user_guide/module_segmenteditor.html
In the official website there are some tutorials that can guide you toward the basic functionalities. I suggest you to start from the Segment Editor and the Segmentation modules, check the Docu and explore the option on the software.
YouTube is also a good tool to fastly learn some basics of 3D Slicer usage. You can check this channel, also linked in the official 3D Slicer website: https://www.youtube.com/user/PerkLabResearch/feed
Other than that, i used 3D Slicer for my graduation thesis research, in which you can find an use case for bone segmentation with Slicer (chapter 4).
Thesis Open-Source Workflow for the Use of 3D Printing in Dentistry