[Isofarro]

Since you've done PHP and MySQL previously, if you still remember how to use that, it's worth starting with a simple PHP micro framework (like Slim, or maybe Symfony 3). That will teach you about using URLs for routing. (If starting from scratch, pick a language, and pick a micro framework for it: Ruby => Sinatra, Python => Flask)

Get comfortable with that, and then create a route that sends back information in JSON instead of HTML. That will then give you something to try Ajax against.

Also, learn CSS. The Web switched to CSS for layout in 2001-2004. Have a read through 24ways.org -- their advent calendar for each year, it's nice bit size bits about modern web development. CSS for layout. Responsive design (basically adapting to various browser window sizes and screen resolutions). If you want to learn how to lay out a page in CSS this is essential, but if you want to get up to speed just building stuff, then taking something like Twitter's Bootstrap is a handy starting point.

Also, jQuery. Learn about using events properly, and keeping JavaScript in a separate file and hooking into the browser/document events you are interested in. jQuery is the starting point for getting your head around Unobtrusive JavaScript.

If you are interested in getting your head around static HTML/CSS sites first, it might be worth pulling up a Jekyll tutorial and following that. (Jekyll is a static site generator. Markdown documents rendered as static HTML using the Liquid tempting language)