The most obvious choice for any skp conversion would be to use SketchUp, because it is the program that natively uses .skp files. Unfortunately, SketchUp does not support STL format by default, which makes skp to stl conversion a little tricky.
What you need to do is to have a proper plugin installed in SketchUp, for example SketchUp STL and SketchUp to DXF or STL plugin should be sufficient to convert any skp to stl.
In case you do not have access to SketchUp, you will have to rely on some 3rd party alternatives that offer support for SketchUp formats.