It's just not good as a learning tool. I'd recommend learning elsewhere and using Duolingo for reviewing or getting familiar with the language. (and only if you like it)
Do you know the kana already? Those should be your priority nr. 1 priority in whatever you do.
Input is also very important. Fastest way is to start using Anki deck(free) or Wanikani(paid) so you'll get past the kanji barrier. Learn grammar from a textbook or alternatively Youtube channels like TokiniAndy and/or CureDolly. If you like games, Game Gengo is an excellent channel as well.
Are you a native English speaker? If not then you might want to pick up a Japanese textbook from your local library as some things are much easier to understand through your native language.