You have to set the PATH environment variable to include the path to javac. See
this Q&A section on the Java Tutorial.
Are you sure you want to be writing an Applet now? They are coming under pretty heavy attack - and most browsers, companies, and even Homeland Security are suggesting not to allow Java plugins to run in your browser. Why not learn Java on a desktop app? I don't mean to dissuade you from learning Java, I just want to make sure you think about it before you concentrate too much on Applets only to find out they aren't a good idea.
And Welcome to the Ranch!