Hiragana (ひらがな) is used mainly for grammatical purposes. While katakana (カタカナ) represents the same sounds as hiragana, it’s mainly used to represent foreign words. Foreign names are represented in katakana, as are many foreign foods. Kanji (漢字) are adopted Chinese characters used in modern Japanese writing. Most Japanese words (nouns, adjectives and verbs) are written in kanji. There are no spaces in Japanese, so kanji helps distinguish when a new word begins. Kanji characters are symbols that represents words. So, start with Hiragana and after it with Katakana. After you mastered these, you can go on with Kanji.

Thanks :)

In Japanese, Hirakana, Kakana, Kanji are used equally. Philip von S.'s answer explains their uses exactly. Actually, Kanji is useful in East Asia. You can really stay in East Asia only by Chinese Character (Chinese name: hanzi, Japanese name: kanji, Korean name: hanja).