If Windows, find the directory where java.exe and javac.exe are and make sure this directory is in your PATH. Also make sure that the directory that contains the source code files that you are working on is in your CLASSPATH. (Various forum threads explain how to do the PATH and CLASSPATH things.)
Use Notepad to edit your source files. If you are working on myjavafile.java, then compile it by typing "javac myjavafile.java" and run it by typing "java myjavafile"
Hope this helps.