Assuming you are using windows, open the document in SketchUp, go to file, go down the list to the submenu Export, select 3D graphic in the save window that opens go to the file type menu under the file name box; in that menu, you will find STereoLithography File (*.stl). Select that and click Export. Your file will be ready in a few minutes.
If you use Mac, I'm not sure how the process goes.
I hope this helps.