Create Servlet file and study web descriptor file
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.1" xmlns="http://xmlns.jcp.org/xml/ns/javaee">
<servlet>
<servlet-name>WebServlet</servlet-name>
<servlet-class>WebServlet</servlet-class>
</servlet>
<welecome-file-list>
<welcome-file>MainPage.html</welcome-file>
<welcome-file>index.html</welcome-file>
</welecome-file-list>
<servlet-mapping>
<servlet-name>WebServlet</servlet-name>
<url-pattern>/WebServlet</url-pattern>
</servlet-mapping>
</web-app>