Maybe you should start with coding after all
Since I started to learn marketing, I've been given the same piece of advice over and over. I've seen it on Twitter, YouTube videos, books and conference talks:
Build your landing page first!
"But where am I going to get traffic?"
If you can't figure out how to get traffic to a simple landing page *now*, how are you going to do it in the future once your app is built?
(That's a tweet by Justin Jackson btw)
As with everything in life, this advice is right to a point. For its intended audience, developers who go straight up to coding in a five-month project without doing any validation first, it's an awesome advice.
For makers, it's a different story. The maker culture urges you to keep shipping, even if your project isn't ready yet. So you're already doing that outreach and audience building you'd do with a landing page.
So what would I recommend to my fellow makers? Use the time you'd spend in creating a landing to build a minimum version of the core feature of the app you wanna work on, then share that. You'll not only be able to measure interest but get people engaged and also gather some early-stage feedback.