The western alphabet also consists of two "alphabets": capital letters and small letters, though we're so used to them we don't perceive them as two seperate alphabets. Think of Hiragana as small letters, and of Katakana as a cross between capital letters and italics. Katakana are used for foreign words and to emphasize something, just like italics are. Think of Kanji as pictures that express meanings or ideas. They were originally real pictures, but have become really abstract over time. They also have sounds (originally Chinese) associated with them, and Japanese has imported a ton of Chinese words just as English has imported Latin and Greek words. So there are two fundamentally difference ways to use Kanji: You can express a native Japenese word with the meaning of the Kanji and add an ending in Hiragana, or you can express an imported Chinese word by the Kanji for the syllables of this word (and of course the Kanji will also be related to the meaning). The first is called Kunyomi ("meaning reading"), the second is called Onyomi ("sound reading"). In English, it would look like this: If 生 is the image for "live, life", instead of "He lives in Japan" you'd write "He 生s in Japan" (and read it as before). OTOH, you'd write "biology" (life-science, "bios" is Greek for life) as "生学" and read it as "biology". (The actual Japanese expressions are a bit different, instead of "He lives in Japan" you'd say "He resides in Japan" and use 住む). So people will generally use Kanji in this way as far as they know the Kanji. They'll write endings and common words in Hiragana, and they might also write words in Hiragana if they don't know the Kanji for them. They'll use Katakana for foreign (English, Dutch, ...) words and to emphasize words. It might vary a bit from person to person, but on the whole, it's consistent.

We use these three, we can't write anything with using only kanji. Learning hiragana first is needed, if you use hiragana perfectly, you can write the short and simple Japanese sentence. Next, you should rememver katakana because we use it to write the name of foreign coutries and people from overseas but there are some exceptions. We also use katakana to write the Japanese words which are originary from other languages. You can communicate without kanji but if you learn them little by little, it'll help you with learning Japanese more. Please enjoy learning Japanese!