I'm not sure I follow what you mean exactly. How do you write in Japanese without knowing the written characters? Normally you would use what's called romaji. Romaji is taking the hiragana and converting it to something you can type on your keyboard. For example, ねこが好きです------> neko ga suki desu. (I like cats.) If you are asking about grammar, or how to organize words to make a sentence, then you will need to either get a textbook or learn the common and basic rules online. It is too much for a single answer to cover.