If you're looking to get into web programming, then HTML is probably the best place to start. Strictly speaking, it's not programming, but HTML will be the foundation on which everything else will be built. If you don't know how to write basic HTML, it won't do you a lot of good to learn the rest.
WebGL would be for games or anything that might require 3D graphics so you can pretty much forget about that, and Adobe is looking to phase out Flash for HTML5, so while there might be a pretty good sized market now, 10 years from now you will probably (and hopefully IMO) have a hard time finding Flash being used anywhere. You can learn it if you like, but I'd make it a secondary task to be undertaken after you've gotten a pretty good grip on the rest.
Somewhere along the way I would also suggest looking at the likes of PHP and frameworks for creating websites such as Cake, Django, and Ruby on Rails.
The single most important thing, however, for any programmer... Do not fall into the trap of learning a particular language, you want to learn the "how" of programming. A good programmer can pick up any language in a couple of weeks because they understand concepts such as iteration, so it's just a matter of how different languages handle the particulars of iteration.