Ahem.
Now with that said, almost every single person who learns Japanese, including native speakers, would probably learn hiragana and katakana first before moving unto kanji. The only exceptions are native Chinese speakers, or people who have otherwise learnt Chinese first.
This is partly because learning kanji is hardly useful if you don't know how to
read them. And since most pronunciations are written in kana...
Japanese has three basic components in its writing system. These are
Hiragana, the Broad-stroke syllabary,
Katakana, the Fragmentary Syllabary, and
Kanji, the Chinese Characters.
Both Hiragana and Katakana are
syllabaries, that is, they are sets of written symbols that represent sounds (syllables in this case), similar to how the Latin alphabet (ABC) is used for both English and French.
Kanji, on the other hand, is
logographic -- they represent meanings instead of sounds. Instead of spelling out "D-o-g" or "É-t-o-i-l-e", for example, these words would be written as 「犬」 and 「星」 in kanji.
Kanji is generally used for "concrete" words, such as nouns, verb stems (the 'root' part of a word, like "swim" for the English "swimming"), adjectives, and adverbs. In a sense, kanji are the building blocks in your average Japanese sentence.
Hiragana is usually used for grammatical particles (like the word "to" in English), verb and adjective inflectional endings (like the "-ing" in the word "swimming"), and exclamations (Like the English "Ah!" and "Hey!'). Hiragana can be viewed as the "glue" which bind the "meatier" parts of sentences together.
Hiragana can also also used as furigana, in which case they are often found on the top of kanji as a pronunciation guide. Some words with obscure or hard-to-read kanji are also more commonly written in hiragana.
Katakana is most commonly used for loan-words, or words borrowed from a foreign language. "France", or example, would be written as 「フランス」. Foreign (non-Chinese/Korean) names are also written similarly, like 「ジョン」 for "John".
Onomatopoeia, or words that imitate sounds, like "oink", "meow", and "boom", are also written in katakana.
Katakana can also be used for emphasis (similar to
italicization or writing in ALL-CAPS) Some animal or plant names with obscure kanji are also written in katakana instead of hiragana, like 「バラ」 for "rose".
This article may be useful if you have any further questions.