Well, as someone who's learned it... :) and a couple other languages... let me just say that the best way to learn it is to have a Japanese boyfriend/girlfriend.

Japanese is unique in that the book or school type stuff you'll find online are barely used in regular conversations.

If you do learn online or from books/school, people will be able to understand you, but you'll never hear people actually talk that way unless it's an old woman at a formal event.

Try Mahalo's guide to How to Speak Japanese.

In addition to steepdecline's answer, the best way is by listening to or using it, but it really helped my daughter to have some formal instruction first. It helped her to start with some vocabulary and basic structures to go off of. She said, the Japanese language likes to shorten words and structures often, but many times, with the proper ear training, the original is pretty easy to figure out for her, if she knew it in the first place. She has had a mix of class time and Japanese media aka anime (and friends/conversation partners). Our family friend got a brief crash course before spending a year there. After two years back here, she's still fluent, and after a year and a half of study, my daughter doing fairly well with at least listening and keeping up with our friend.

Having studied several languages herself, she has been most successful with Japanese because of this mixture. To be honest, this is probably the best way to go about learning any language.

Myself, I speak 2 languages (barely). However, with very limited and almost nonexistent formal study of my parents' native tongue, It is almost gone even when speaking with them. Ex: 6 kids in our family, 4 have had some though small formal teaching in the native tongue. Their speech is holding. The 2 of us who didn't have formal teaching in the language struggle greatly. However, the 4th who had formal training had the training as an adult and sits 5th in the birth order. So there is something to be said about some formal training.