One major difference to other domains of programming is that web programming involves a lot of different technologies, even at the most basic.
At the very least, you need to understand the HTTP protocol. It's the essence of what the web is. Reading the spec a few times might be a start. It may appear a bit dry, being a technical document, but I promise that it's fairly readable. Come back to it once in a while, as you gain knowledge - You'll find new things. Read also stuff about REST - It's a buzz word, but it could be loosely translated to HTTP-used-as-it-was-intended.
Another very important technology is the browser. You generally deal with a suite of three distinct languages here: HTML, Javascript and CSS. HTML is by far the most important to understand and it also forms the foundation for the other two.
On the server side of things, you need to deal with language choice, web servers and databases. On top of this there is frameworks that ties it all together. You can program web applications with very minimal frameworks (plain PHP at the extreme end) or with very sophisticated (and complex) frameworks. I'm of the belief that starting with something minimalistic is better, since it'll make it easier to learn. It doesn't have to be PHP though; ASP.NET may be fine (I don't know it first hand, so I can't really recommend on that) or perhaps Ruby on Rails.