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).

It is quite unfortunate that I know how daunting it can be to venture into this world.

If you already have some experience (as you mentioned in another comment) in whatever language it is, stick to that language. Find a good introductory book / tutorial that explains data structures. Whichever introductory book you find (I'm biased towards books - but each person learns a different way, whatever works for you, go with that) will teach you enough to UNDERSTAND what you are coding.

You'll learn how each line relates to the other. You'll learn how to manipulate that which you are building. How one thing relates to the other within the constraints of certain laws which were created to be able to communicate with the machine.

As you progress, you'll see how other people have already done certain things so that they won't need to repeat themselves later on. These functions, whenever available, can be used by anyone.

When you learn data structures, you can grab these functions to ease your own coding. But to be able to use them, you'll need to know how to manipulate the instructions that are contained within them.

I digress. As I mentioned earlier, I'm new at this. If I'm wrong I am wrong. If I explained stuff you already know, oh well. But it does help myself get a firmer grasp of what I may or may not know.