If you are using a commandline in a console window, you need to be in the correct directory relative to the .class file and use the java command with the classname. If the class is in a package it is more complicated.
For a class not in a package, change to the folder with the .class file and enter:
Also enter the dir command to show what is in the folder.
To copy the contents of the command prompt window:
Click on Icon in upper left corner
Select 'Select All' - The selection will show
Click in upper left again
Select Edit and click 'Copy'