I was wondering, what do I need to know to create professional websites, other than HTML and JavaScript?

Define professional.

I run my companies website with just HTML, Java, PERL, and Flash. More advanced sites require SHTML, XML, ASP, and PHP. What are you trying to design?

Well I would like to go to school for web design and stuff, because I love computers. . And I understand html and haven't really looked at learning any other stuff yet. . But I was wanting to get a head start on things so that when I start my classes this fall I will understand the material better. If I understand html, will I most likely be able to understand other codes as well?

HTML, SHTML, and ASP are all pretty similar. XML is like HTML, but is just for transferring variables, so here is a lot less coding to it.

PERL is really close to C. Java is not quite as close, but similar.

Flash is almost like Visual Basic, but a lot easier. I don't really have any experience with PHP, but I understand that it follows a lot of the same rules as PERL and HTML (Don't quote me on that, I've never used the language)