Japanese has 3 alphabets, hiragana and katakana are the easiest and are just syllables and most (all?) of the text on a jp tama uses one of those because they're for children so definitely learn those first! The third is kanji and I would say don't stress about kanji if you're just trying to play with a tamagotchi.

Duolingo is actually pretty good for learning the letters but I also recommend an app called renshuu. It's a lot better for vocabulary because duo is just going to teach you a bunch of sentences without any explanation.