Welcome to the Ranch
Any suggestions about setting a CLASSPATH are likely to be unhelpful. In fact a system CLASSPATH can do more harm than good. The sort of error you quote suggests you have nothing wrong with the Java™ installation, but there is a problem with your own file.
You are not clear about what you are running. Are you running a .jar or a .exe? If a .jar, what is in its manifest file? Please print that out. What is the fully-qualified name of the class with the main mathod in?