Having trouble compiling Helloworld Java program?

Well, I've installed the Java SDK and Eclipse, and everything works fine. But, I wanted to learn how to compile a simple program written in a text editor via the command line. I've gotten the command line to the right directory to the Helloworld.java program. But when i type javac Helloworld.java, it says javac is not recognized as an internal or external command. Why is this? What am I doing wrong? Thanks.