Which software tool would you recommend for the beginners of 3D printing and why?
Really depends. If you’re buying a printer kit, start with the recommended software from your kit supplier to get familiar with how to operate it and basic concepts. For cad modeling I’d recommend http://tinkercad.com, for more organic modeling… Rhino, blender or 3dstudiomax
If you are going to use it to edit STL files take a look at trueSpace. Has the ability to get rid of all the triangles on the polygon faces making it really easy to edit the files. Similar to Blender and 3dS.
TinkerCad, cura, thingiverse.