What is needed to run Java Servlets ?

Hello, Iam in my first steps in Java Language, and Iam asking about what is needed to program and run Java Servlets,I know that I need the JSDK and the JDK, but there r some settings I think should be altered like the CLASSPATH or something like this… Iam not sure , I need full information about this subject ..
Thanks in Advance
Miss.Hanan Khader