What's the difference between hiragana, katakana and kanji?

Kanji carry the meaning of most nouns, verb stems and adjectives. Hiragana writes grammar: particles, verb endings and words without common kanji. Katakana writes foreign loanwords, names and emphasis. A normal sentence uses all three.

One sentence, three scripts

私はコーヒーを飲みます。 “I drink coffee.”

Part	Script	Role
私 (I)	Kanji	Meaning
は	Hiragana	Topic particle
コーヒー (coffee)	Katakana	Loanword
を	Hiragana	Object particle
飲 (drink)	Kanji	Verb stem
みます	Hiragana	Polite ending

Why not one script?

Japanese has no spaces between words. Kanji show where content words begin, hiragana shows grammar, katakana flags foreign words. Japanese also has many words that sound alike; kanji tell 橋 (bridge) and 箸 (chopsticks) apart in writing, both hashi.

The order to learn

1. Hiragana: 46 basic characters. A week or two of practice.
2. Katakana: another 46, same sounds, different shapes.
3. Kanji: a long project. The jōyō list has 2,136; the JLPT N5 level needs about 100.

Starting kanji before kana makes reading impossible, because every sentence needs hiragana.

Where mnemonics help

Kana are few enough to drill. Kanji are where memory tricks pay off. Kanji Mnemonics (my app) teaches each kanji with a keyword, a short story and a picture, level by level from N5. Lesson 1 is free.

Related: how to learn kanji as a beginner, read or write kanji first and the Kanji Mnemonics guides.