Java itself is not a web technology. There are some sub-sets of Java that are useful in developing web sites such as
JSP and
Servlets. What technologies you need to learn depend on what kind of web site you want to make. Really HTML is all you need to know to make a basic web site. Other technologies can help make your site more dynamic and interactive, but again, it depends on what kind of site you want to make.
Also, this question would be better suited for the HTML and JavaScript board. The people there are more into the web development stuff than on this board.