Start by memorizing the hiragana syllabary (basically one of the Japanese "alphabets"). There are 46 letters in this syllabary, and each one corresponds to one sound. Once you have memorized these 46 letters, you will be on your way to learning and speaking Japanese, and you will be able to read many signs, menus, children's books, etc... From there you can begin learning basic words, and very basic grammar that will enable you to understand simple phrases, i.e. I like this, I don't like that, I want this, I want that, I'm happy, I'm sad, etc...
It's hard to learn the basic grammar without having a firm foundation in hiragana. Once you've got it down, you are off and running.
There is a corresponding syllabary of another 46 letters which are kind of derived from hiragana, called "katakana". This alphabet is used primarily for words that are foreign in origin. So many words relating to computers, technology, foreign foods, foreign clothes, etc... are written using katakana. For this reason, many foreigners find it easy and satisfying to start off their Japanese language study by memorizing the 46 katakana characters first. However most texts and most instruction methods recommend you memorize hiragana first.