Android apps are written in Kotlin, so this is the language you'll use in every module that follows. You don't need to learn all of it before you start building, but you do need a solid grip on the core: how to store data, make decisions, and organize code.
Start with variables and types, control flow, and functions. Then move on to classes and interfaces, which is how Android code is structured, and to Kotlin's signature feature, null safety, which prevents a whole category of crashes. You'll also meet lambdas and extension functions, which Jetpack Compose uses heavily.
The lessons below are full-length courses, so you don't have to watch them end to end. The first one covers the core and is the best place to start; the other two go deeper on the same ground. When you're comfortable, continue on to Version Control & Repo Hosting.
Philipp Lackner covers variables, nullability, control flow, functions, lambdas, classes, interfaces, and generics in one focused course. Start here.
A long freeCodeCamp course from Alexandru Cristian that goes from setup and syntax through object-oriented programming, functional programming, and generics.
A thorough, slower-paced walkthrough of the language from Donn Felker, useful if you want extra practice on the core concepts.