What programming language should I start with?

The development environment is easy to find, obviously...any browser and any plain text editor. But user interaction is hard, and gets all mixed up with nasty DOM and browser incompatibilities in short order. You also have to have additional tools (like Firebug) to get anything resembling useful errors from JavaScript.

A good modern learning language probably ought to have an interactive shell. Ruby, Python, Scheme, and Smalltalk are probably all good choices. JavaScript can have an interactive shell in the form of FireBug...but again, it's scary when you get to doing anything that interacts with the user and very few tutorials of JavaScript cover interacting with JavaScript interactively. Having the knobs do something immediately when you twiddle them is almost certain to speed up the learning process. The human mind is pretty good at abstract reasoning: "flipping this light switch causes the light to come on over there". But the more you separate things out temporally and the more variables involved the less they stick: "flipping this light switch starts a timer, which depending on the current state of the light, will cause it to turn on or off in three minutes".

Merely being a non-compiled language is still a slower iterative process than direct interaction (which is the primary disqualifying point against Perl as a learning language in my opinion...corrected in Perl 6, of course, but that's still a few months out). The shorter the "try, debug, try again" cycle is the faster one picks things up, I think. I found Ruby very fast to learn, to some degree, for that reason. Python was almost as discoverable, though I found it less intuitive, where "intuitive" is probably defined as "kinda like what I'm used to". PLT Scheme was a pretty cool learning experience, but it became less interesting as soon as I wanted to build something useful with it--so few libraries (of course, as a Perl user, I feel that way about every language other than Perl...CPAN spoils you for all other languages when it comes to "just get it working" prototyping).

Down that path lies madness. I've seen the newbie-focused tutorials for PHP, and it explains a lot of the garbage that passes for software in the PHP world. It is entirely possible to write good software in PHP (I'm reasonably impressed by the Dokuwiki and Flyspray code, for example), but you have to learn the language at least twice to get to that point. You learn it as a newbie, and learn horrible practices from the vast collection of misguided "help" in the Internet. You produce lots of code with those horrible practices...like no DB abstraction, inappropriate or no use of objects and encapsulation, and "gradually mix PHP into your HTML" design. And then you realize you've built an unmaintainable mess and start rewriting it with all of the better practices that have since been bolted on to PHP.

Of course, despite all of that I still recommend PHP for a large classes of application because it is pervasive, hard to make pathologically slow (whereas Ruby and Python can be made pathologically slow even by really good developers--see Plone for an example), and has a pretty large selection of libraries (some are so bad as to be counter-productive, but many are good). But as a newbie language it's rather toxic.