It's hard to offer more specific direction without having at least a vague idea of what type of web app you're trying to build. If you're looking for a "read X book" or "do Y tutorial" type of answer, there simply isn't one, not for what you're trying to achieve. The answer is, learn as much as you need to in order to start building stuff, even if you don't know what the hell you're doing at first. When you get stuck, search around on StackOverflow and elsewhere; if that doesn't work, seek out help from your local Python/Ruby/whatever Meetup and try to find other people willing to teach (you seem genuinely curious and respectful so that should make things a little easier). Just remember that you're a smart person, so when (not if) you're made to feel like an idiot for not knowing some silly basic thing, don't back down. Keep searching, keep building, keep asking questions until you get there.

I'm far from an expert but I've been through this. I was in a very similar position to you less than a year ago and have since then gone from absolute zero technical knowledge to becoming a shitty-but-enthusiastic self-taught hacker. I'm happy to chat more specifically if you want to message me directly (first name: Linda; last name: last 4 letters in my HN username; email: firstlast@gmail).