Sources like YouTube provide in depth guides for exporting your CAD files to STL format. Listed below are some major 3D modeling software examples:
Autodesk inventor: Save copy as > select STL format from drop-down.
Standard Google SketchUp: Download STL exporter from google warehouse
Solidworks: Select File > Save as, then export as STL format.
Pro/Engineer: Select File > Save a copy > Type > Select STL format.
AutoCad: (Only works for 3D model) Select File > Export > Select STL format under options.
Rhino: File > Export selected as STL format > Select Binary > Under mesh options more details, Set max angle = 20, max aspect ratio = 6, minimum edge length = 0.0001 > Done