There are many ways to learn Japanese or a new language for that matter. One of the many ways that people seem to learn the best and the fastest way is to immerse yourself into the culture of that language. It's also worth noting that the earlier you start to learn a new language and develop it over the years, it becomes a lot easier to learn. Here are some examples to help see with one will be best for your needs.
Try listening to Japanese music and pay attention to the lyrics on what they say. Sometime familiar phrases or words may be repeated in songs you hear. Like any song you want to learn to sing, you have to remember the lyrics. When learning the new language, you'll learn some new words to remember. So keep an ear out for some words you've learned and pay attention to them.
Watching movies can be a good attention getter. If you watch a foreign film or show you own, watch it in the native language with subtitles. With subtitles on, pay attention to those words and what the people are saying on screen and compare some repeated words to the translation. For example in Japanese, when a character says "nani", and the subtitle reads, "what", you'll get the cue that's what the word translate to. Like music, pay attention to repeated words and phrases to know what each word is in that foreign language.
Nothing more basic than using your memory to help learn a new language. While simply remembering something won't cut the job right away, here's a basic way on how to remember words or the alphabet. Take some flashcards and write on one side the letter or word, and then on the other side write the translation for it. Flip back and forth to try and remember the translation, or the the other way around back and forth. For words you got right, put to one side. Ones you got wrong, put to the other side and practice learning those more. While the most basic, it's the most accessible and simplest way to remember.