Apache Tomcat https://tomcat.apache.org/download-80.cgi
Here is Oracle tutorials https://docs.oracle.com/javaee/6/tutorial/doc/bnafd.html https://docs.oracle.com/javaee/6/tutorial/doc/glrbb.html https://docs.oracle.com/javaee/6/tutorial/doc/bnaxu.html
Tutorials point tutorial https://www.tutorialspoint.com/servlets
One more tutorial https://o7planning.org/ru/10169/java-servlet-tutorial