I built an app myself and made plenty of mistakes.

Before you start paying for ads, make sure you get the basics right. Define a market & strategy for distribution, launch on free platforms like Product Hunt, set up some Search Engine Optimization (SEO & ASO), and try your hand at a bit of viral marketing. These things will help you make sure you're not wasting money.

There are a bunch of free alternatives to Product Hunt that you can also try: Tinylaunch / Microlaunch / Simple Lister / Beta List / StartupBuffer / StartupInspire / Uneed

Source: https://speedbumpapp.com/en/blog/mobile-app-promotion/