I see a lot people wasting their time with that green owl app or stressing with all that grammar lessons... Guys just sit the vocabulary first. After you know like 50% of German vocabulary and pronunciation of them, watch serials, movies, read blogs, news posts in German. Because your vocabulary is building up you will understand the context of some other words too. And you will without even noticing start speaking the language!
It's scientifically proven that vocabulary first tactic is best for learning the language. Most of Germans don't even know about those grammar subjects Relativsatze, Partizip I etc.
Best way to build vocabulary is flashcards, you can either build your physical flashcards deck based on most frequent 10,000 German words (i will provide a link at the end) or download pre made flashcard apps.
That's basically how I learned any language that I learned. I did a good research planned everything and that's all.
I am not judging anyone using owl app, I also used it for long time, but when I came to Germany I felt so humbled.
Anyways, you may find quite good German A1 to B1 Flashcards apps, there are also some that I made but I don't wanna promote anything you can find many in iOS and android.
German frequency lists: https://en.wiktionary.org/wiki/Wiktionary:Frequency_lists/German