What steps do you recommend a complete newbie to MTG & MTGA take?

I installed the app a few minutes ago and played through the first 2 tutorials.

I looked into what to expect next and apparently there will be 5 different colour decks, unlocking cards by redeeming and playing matches, buying packs and building decks.

It's all quite confusing to me, so can anyone simplify this for me? I'm a yugioh player, if that info helps and a terrible deck builder. Also, the blue and green colours seem great to me, how good or bad of an idea is it to focus a deck around them?

Although initially I really wanted to go all black because of a manga on MTG I'd once read.

I wanna build a really competitive deck eventually and don't wanna spend any money in the process. That's the goal.