It all depends on what you wanna do. To start off, to the person that said Java programming. No, just plain no.
HTML will build the framework of the site.
CSS will style your site
JavaScript is a browser-side scripting language used for various things. JS is NOT needed in all websites, but it is a pretty handy thing to have down.
PHP is definitely a big one when making a dynamic site. Forums are almost all in PHP, but anything can be made in PHP from logging in, to keeping session data / cookies and validating forms. PHP is something I would spend a lot of time into learning, because good PHP can make or break a site.
MySQL is a good thing to know, so you can establish a connection with a database. Used with PHP to see if usernames exist, create new usernames, logging in etc. Very helpful.
Everything else is optional. Remember, that .php files can contain html elements, so really a dynamic site shouldn't have .html or .htm .php can cover it all.