Depends on how well you know HTML and CSS and only you can really answer that. It also depends on what route you want to go down as to what would be best for you to start learning as other choices include (but not limited to) Javascript, Flash, PHP, MySQL and Java.
PHP is the most useful thing ever. Specifically I would learn PHP includes, and how to manipulate mySQL databases with PHP. These 2 things alone can make your life much easier, and your site extremely effective
Well, instead of diving into server-side stuff (PHP, ASP etc). I would make sure you REALLY know your HTML and your CSS. Also, get Photoshop (but not Dreamweaver, unless you really want it), make up some web graphics, and see what you can put together using just the images and css/html.
Another vote here for PHP - I only know a little bit and it's helped me a lot in setting things up and doing soem interesting stuff
Depends on what you want to do? There's a lot to learn and you would need few dozens of lifetimes to master everything. JavaScript/AJAX, PHP/Ruby/C#/JSP, SQL, ActionScript, Flash, XML (well this is useful with almost any server-side laguage), maybe Apache or IIS can be useful, SEO, graphics (i.e. mastering hotoshop, Illustrator and so on) and many more. Finally, (X)HTML/CSS alone is an area that takes years to master at least partially so I have a feeling like you don't know it as well as you should (not to be rude but judging from your sites in sig, you have a LOT to learn in HTML/CSS area) Just don't stop learning new stuff, once you stop, you will fall behind competition and it will be a hell of a job to get back to it.
php and sql are really useful languages but if you want to take baby steps try javascript although it is getting a little outdated
I dont think so!!! Man, PHP is as easy as HTML... I feel javascript more hard And btw, after HTML and CSS.. you should start PHP, MYSQL and then you can go for JAVA