Start with their greetings?

Ohayo, Konnichiwa, Konbanwa! 8 tricks to greet like ...

Duolingo is an ok app for starters, but later on it depends on how you approach learning. I prefer learning the grammar rules; duolingo isn't that good for that, but people make up for it via the comments section

I recommend learning their alphabet first. The really good books on Nihongo will required you to be able to read their alphabet (Hiragana and Katakana)

I've been self-studying Japanese for 2 years, and I'm still quite bad at it, haha. Grammar is hard, but I'm happy to say I can read text decently so I can atleast understand a little bit.

whoah... harder than i thought i guess XD gadgenista

try duolingo

You can learn the alphabet in a week if you really want to do daily writing drills to help remember