Sitepoint offers their own courses online (learnable.com).
You can also use Lynda.com or receive some training in your city/town about the technologies used in web development.
As @Stevews; suggested, if you're serious, HTML is only a start. But you can learn all of it on the web either by yourself or through online courses. Obviously, books help a lot too.
Sitepoint's book are easy to follow and these are the ones I know best, but there are whoooole range of books that are great for learning and most of the major publishing houses such as O'Reilly have their share of decent books to choose from.
Obviously, the forum can help to answer specific questions (either by looking at answers when that question has already been asked, or by posting your own thread when you can't find that answer)
As with many things in life, it is a question of practice.