error in java progran at run time

Hello sir,,,
i make a servlet program in java,,but i face some problem,,so please me..
Actually this type of error are come in my all servlet program

My Servlet--





Error-
package P1;
import javax.Servlet.*;
import java.io.*;
public class FirstServlet extends GenericServlet
{
public void service(ServletRequest req, ServletResponse res)
throws ServletException,IOException
{
PrintWriter out=res.getWriter();
out.println("hello world");
}
}

Error-
D:\Servlet>javac FirstServlet.java
FirstServlet.java:2: package javax.Servlet does not exist
import javax.Servlet.*;
^
FirstServlet.java:4: cannot find symbol
symbol: class GenericServlet
public class FirstServlet extends GenericServlet
^
FirstServlet.java:6: cannot find symbol
symbol : class ServletRequest
location: class P1.FirstServlet
public void service(ServletRequest req, ServletResponse res)
^
FirstServlet.java:6: cannot find symbol
symbol : class ServletResponse
location: class P1.FirstServlet
public void service(ServletRequest req, ServletResponse res)
^
FirstServlet.java:7: cannot find symbol
symbol : class ServletException
location: class P1.FirstServlet
throws ServletException,IOException
^
5 errors