servlet and error messages

I keep getting the following error messages:

package javax.servlet does not exist
package javax.servlet.http does not exist

when trying to run a servlet on an HTML program that I have written. I'm not sure if its a classpath problem or something I have not installed. Could you help me please

A-Track

Make sure the servlet.jar file is in your classpath.

thank you for your reply.

I had amended the classpath to contain that file already, but I'm still getting the same error.
please help..