Under Windows, Goto control panel, system, advanced, environment variables. Path and classpath will probably already be under system variables, you can edit these.
classpath maybe set to "." which means the current directory.
you can also set classpath at runtime when you run java.exe by using the command line switch "-cp [classpath]"
If your project uses external jars, you may need to set this switch when you use javac to compile your classes so javac can located the classes within the jar file.