You really have two choices:
ASP.NET or
PHP. ASP isn't supported any more and is, frankly, horrific to work with, but ASP.NET with Visual Basic or C# (you need to use it with a .NET language - ASP is just a framework) is actually quite nice. The other option is PHP, my preferred choice.
There is a third choice - a relative newbie to the race called
Ruby on Rails. It uses
Ruby and gives you a pretty cool framework to work with. I'm just about to start learning it myself, so I can't really offer my opinion on it, but it looks pretty easy to learn and provides you with a lot of backbone from the offset.
Which one you pick is largely down to personal opinion. Learn about both and decide. PHP and Ruby will work on more servers, being open-source and available on all major operating systems, and both ASP.NET and Ruby give you more of a structured framework to start with. It's worth noting, however, that PHP frameworks can be found that give you a lot of the same structure -
CodeIgniter's my favourite.
You also need to learn about databases. If you're using ASP.NET, I'd recommend
Microsoft SQL Server - they interact quite well with each other. With PHP and Ruby,
MySQL and
PostgreSQL are usually recommended - pick one.