Modern web development is actually broken into multiple facets and technologies, and they all compliment each other..
You have the server front-end components, usually just a web server like Apache.. The back-end components, which constitutes a data storage, often some kind of Structured Query Language-based database engine (typically MySQL when using PHP, but there are plenty of choices), and the scripting engine (like PHP) to retrieve and structure the data in a useful way to return it to the web server, which returns that page to the client (web browser), normally this will be HTML, XHTML, or some kind of XML-based abortion.
So that returns the pre-laid-out data to the browser, and usually applying a standard stylisation template using CSS reference by the programmed page returned to the browser, so styles are consistent across pages. And when you want to manipulate the data on the web browser itself, you use JavaScript, which can even update pages without reloading the page using the Document Object Model (DOM) and the XMLHttpRequest Object, colloquially known as AJAX (Asynchronous JavaScript and XML), due in part to its customary XML (or JSON (JavaScript Object Notation)) data interchange between the web server, its back-end components and the web page.
It's a bit of a messy web of technologies, but with practice you'll get the ins and outs.
The best place to get started is here:
http://www.w3schools.com/
Play around with HTML, try using a CSS template for styling HTML objects, then you could probably get started with a little programming with javascript, then you can start learning some PHP and MySQL programming, which is a fair bit heavier and more involved.
When it comes to the PHP portion of the learning, it would be a good idea to use an Apache/MySQL/PHP bundle like XAMPP, which avoids having to fiddle around with configuring the individual components yourself:
http://www.apachefriends.org/en/xampp.html
And of course, if you ever get stuck on something, you can always ask here.