I know a lot of people have given great advice here already, so my only advice would be:
- Learn HTML+CSS+javascript. They are the backbone of the Internet from a client perspective and ultimately you need to know what the higher level languages and frameworks are going to emit.
- Understand what is going back and forth across the wire between client and server.
- Start thinking of decoupling systems.
- Understand how relational databases work.
Once you've got this all in hand, you're much better positioned to choose a higher level language/server architecture/framework.
ASP.net is good, but hides so much that unless you really know what you're doing, you can really create a 'heavy'/slow site. Particularly if you don't know what AJAX is doing under the hood, you can easily create a slow site. Still, obviously Visual Studio is awesome, and if you know C#, you can just get on with making a site without too much trouble. I would likely start with the newer MVC way of doing things than the Forms approach, for lots of reasons that you can read about. Just my opinion.
PHP is good and incredibly successful, but I found debugging to be tedious, although I believe there are so great tools here from Zend which help, but I haven't used them. I just logged everything (like the good old printf() days ;))
I think one other option which could be good for you, is to try out Google App Engine (GAE), despite it's various gotchas and issues, with GWT (Google Web Toolkit). I've found this an excellent environment to work, because you don't have to worry about servers (Apache/IIS) or traditional databases (SQL/MSSQL), you just write in good old Java for the backend, and also Java for the client side, which emits very optimised javascript. Plus you can work inside Eclipse on Mac/PC (I'm on Mac nowadays), with full debugging etc. You will be forced to think in a distributed manner right from the start, including database structure, so you'll build good habits. It also supports Google accounts sign in, as well as memcache and session state. Really, I think it's an excellent solution and could be a great way to get started.
Oh, and you can also code in Python if you like on GAE, which is nice because you can try out two new languages easily.
That said, if you're looking for a job in this business, you'll probably need ASP.net, so you might not have much choice. Just make sure you understand the points 1.2.3 above.
Anyway, good luck. I was in your position a year ago, and I tried it all, but ultimately settled on Google App Engine+Java.