WOW .... fun question!
If i had it to do all over again, I would choose either python or c# to start.
First ... They're both free to use and quick to get started with.
Second ... There are TONS (specially with python and javascript) of ways to get in
Third ... You could build something cool or useful on your first day.
There are a few factors that would make up my mind
1) What do you want to build? Are you a server admin with an eye for automation? are you a designer that really wants to make an interactive web piece? Are you just in it for the theory? Do you want to build the next big app without having to trust a stranger with your most precious idea ?
C# is a great language and if you're using windows I'd recommend it to start. Javascript uses a very similar syntax so whatever you learn you'll be able to apply it to both sides of a web project or develop a windows desktop application.
-- there is a great IDE called SharpDevelop that's open source and
really fast to use or there's the
express version of Visual Studio.
C# is my number one recommendation because VS and SharpDevelop both have EXCELLENT support >for code completion. (which means that you don't have to know what a thing does to use it >.... just start typing and the system will start making suggestions.
Python is the best supported from a documentation perspective, and it's got this thing called "interactive mode" that lets you try things out REALLY EASILY! you can do things like typing "1+1" and it will respond!
Python is also great for buiding just about anything. There is a TON of code out there you can use and you can even use google app engine.
If you're looking for a quick to use
and VERY POWERFUL (did i mention fully
cross platform) language to build web
and server apps or to automate
management stuff .... this is your
best bet.
If you want to build apps that run in the browser you need javascript. The down-side is that browsers are terrible at telling you what's wrong, and what's right in one will be wrong in another ... add sub-standard support for a number common language features and it's really not the most fun place to start.
hmmm .... i think i'm off to read up on Erlang .... this has me all excited!