"Web app" is pretty broad spectrum, from things you can do manually (or with very little learning) with an online form tool, to "we have 20 programmers working on this for a year."

The simpler the idea, or at least the simpler the first iteration, the faster you'll be able to learn the necessary skills. You can learn them, it's just a question of how important it is that you do, and how much you have to learn.

So maybe start with "what's the simplest thing I can build, and how can I prototype it". Something like airtables might allow you to prototype easily, though perhaps not go live.