If you were to pick one to learn first, I would recommend CSS.
CSS is not only for "fancy styling", it's for cleaner and more technically sound styling. The benefits of CSS are numerous. If you aren't familiar with XHTML standards, I would recommend learning them too. The standards are easy to learn; it's just a certain way of writing HTML. I second the recommendation of http://www.w3schools.com/ as a place to learn.

Scripting languages like PHP or Perl are not of much use unless you know what kind of HTML/CSS you want to be producing as the final product. To oversimplify greatly, scripting languages are a way to dynamically produce HTML/CSS. Many parts of web pages are duplicate information. A header, a navigation section, a footer. Rather than type those over and over 100 times, you can write a script to produce those sections automatically. Then if you need to make a change to the header section (for example) you change one PHP instead of changing 100 HTML files.

SQL is entirely useless unless you know a scripting language. PHP or Perl or Ruby or some other language are required to take data out of a database and display it.