The workbench you will start using in FreeCAD depends on the type of job you need to do: If you are going to work on mechanical models, or more generally any small-scale objects, you'll probably want to try the PartDesign Workbench. If you will work in 2D, then switch to the Draft Workbench, or the Sketcher Workbench if you need constraints. If you want to do BIM, launch the Arch Workbench. If you are working with ship design, there is a special Ship Workbench for you. And if you come from the OpenSCAD world, try the OpenSCAD Workbench.