1. Katakana for foreign modern words or to emphasize words (think upper-case letters, or italics). 2. Hiragana for word endings, words that are usually written without Kanji, or complete words when you don't know the Kanji (think lower-case letters). 3a. Kanji replace the beginning of "native Japanese" words. The word ending is written in Hiragana, so one can still inflect it. This is usually the "Kunyomi" reading of the Kanji. 3b. For "Chinese-style" words, (usually) several Kanji make up one word. This works like words of Greek or Latin origin in English, e.g. biology (= life + study). This is usually the "Onyomi" reading of the Kanji. If you're unsure what to learn first, learn Hiragana. A nice site for self-study: http://www.guidetojapanese.org/learn/grammar/writing

For the first time, you need to learn how to write subject in Japanese. For example, watashi wa= 私は、it means I (am) .　kare wa= 彼は、it means He( is). sore wa=それは、　it means it (is). etc. Please learn verbs after learning subject. Ex, taberu=食べる（eat)、neru=寝る(sleep),yomu＝読む（read), iku＝行く（go)...Pupils are also remembering Kanji at school and home everyday. So you may remember 800 kanji at first. Keep it up!

normally, we use all of them. basic is hiragana. katakana is maily used for words that comes from foreign language. for example コンピューター(conpyutah) for computer