Manufacturer-supplied .STP files can be directly imported into CAD/CAM programs such as SolidWorks.  These files are solid models.

However, I do not have the SolidWorks program, and I find that Google Sketchup is a very nice, simple, and free program for building and exploring 3-D models.  The Sketchup program is a mesh modeler, which represents models as a collection of points, lines, and planes.

I have found a process for converting .STP files into a format that can be imported into Sketchup.  I found the process online but now I cannot find the link.

Here is the process I have used:

Download and install FreeCAD
Download and install Blender
Import the .STP file into FreeCAD and export it as a mesh file (.STL file)
Import the .STL file into Blender and export it as a Collada .DAE file
Import the .DAE file into Google Sketchup
In Sketchup scale the model by a factor of 0.001 to make it correct.
Now you can save the model in native Sketchup .SKP format