I use Autodesk Fusion 360, which if you are a startup or business making less than a certain amount of money is free to use, and it now includes what were once extras: analysis (FEA), machining, and other features. It really is amazing.
I use FreeCAD, since I have been familiar with it for years. It was once clunkier and had less features, but now it can be used for a lot of the things I need to do.
For a quick start in making a cube, make sure you are in the correct workbench Part or Part Design in the tutorial. The only two issues I have are that it is difficult to interface with clients that use Autodesk, Solidworks, or Rhino products. I have done FEA with the Calculix backend in FreeCAD, and used Paraview to create my FEA pictures for reports. If you know Python, you can even create a cube in the console below. Check out the scripting tutorials. I am not a fan of Python, but I use it in FreeCAD and Blender3D.
