Which reading should I learn first?

The one in the word you are learning. If you meet 山 in やま, learn that. If you meet it in 火山, learn サン (it sounds like ざん there). Add the others as the words arrive.

How do I know which reading to use?

Often you cannot tell from the kanji alone. A kanji by itself or followed by hiragana tends to use the kun reading, and a kanji in a compound of two or more kanji tends to use the on reading. There are many exceptions.

Do I need to memorise all readings of a kanji?

Not at the start. Learn the common ones through example words, and expect to add more over time.

Onyomi vs Kunyomi: How Kanji Readings Work

Short answer: onyomi (on) is the reading that came from Chinese. It is usually used in compound words made of two or more kanji, like 火山 (かざん). Kunyomi (kun) is the native Japanese reading, usually used when the kanji stands alone or is followed by hiragana, like 山 (やま). Many kanji have both.

The two kinds

When kanji came to Japan from China, Japanese already had words for the things they stood for. So many kanji ended up with two sets of readings: one that imitates the Chinese sound (on), and one that is the original Japanese word (kun).

In this site’s kanji pages, on readings are written in katakana and kun readings in hiragana. A dot inside a kun reading marks where the kanji ends and the hiragana tail begins.

Real examples

Look at 山 (mountain). Its readings are サン (on) and やま (kun).

山 alone is read やま. That is the kun reading.
In 火山 (volcano) it is read ざん, as in かざん. That is the on reading サン, with its first sound voiced in the compound.

日 (day, sun) has more. The on readings are ニチ and ジツ, and the kun readings are ひ and か.

日本 is にほん (Japan), with the on reading.
日 alone is ひ (day, sun).
今日 is きょう (today), a special word where the reading does not split by kanji at all.

人 (person) has on readings ジン and ニン, and kun reading ひと.

人口 is じんこう (population), on.
一人 is ひとり (one person), a special case.

休 (rest) shows the pattern with verbs:

休息 is きゅうそく (rest), on reading キュウ.
休む is やすむ (to rest), kun reading やす, with the む in hiragana.

食 (eat) is similar:

外食 is がいしょく (eating out), on reading ショク.
食べる is たべる (to eat), kun reading た, with べる in hiragana.

Rules of thumb

These are tendencies, not laws.

A kanji followed by hiragana (okurigana) usually takes the kun reading. 食べる, 休む, 見る.
A compound of two or more kanji usually takes the on reading. 火山, 学生, 外食.
A kanji standing alone as a noun often takes the kun reading. 山, 木, 水.
Names and special words break the rules. 今日, 大人 and 一人 are read as whole words.

You will find exceptions to every one of these. When in doubt, check an example word.

One kanji, several kun readings

A kanji can have more than one kun reading, and the hiragana tail tells you which one you have.

見 (see) has み.る, み.える and み.せる: 見る (みる, to see), 見える (みえる, to be visible), 見せる (みせる, to show). The kanji is the same. The ending changes the word.
行 (go) has い.く and おこな.う. 行く (いく) means to go, and the kun reading おこな.う belongs to a different word, with a different meaning.

This is why the dot in a reading matters. The part before the dot is the part the kanji stands for. The part after the dot is written in hiragana.

Why there are several on readings

Some kanji have two or three on readings, like 日 with ニチ and ジツ. This happened because the Chinese sounds entered Japanese at different times from different regions. You do not need the history. You need to notice that the same kanji can have a different on reading in different words, and learn each in its word.

How to learn readings without going mad

Learn them in words. The biggest tip. A reading that belongs to a word you know is easier to hold than a reading from a list. Look at 読: よむ in 読む, どく in 読書. Two words, two readings, each with a purpose.

Learn one or two to begin with. The first reading you meet is the one to learn. The rest can wait until they show up.

Do not learn every reading of every kanji. Some are rare. N5 kanji have about as many readings as you will want at first. 十 has several, but you will meet them one at a time in 十分 (じゅっぷん) and 十 (とお).

Say the word aloud. Recognising a sound is quicker if you have said it. Read the example word out loud once.

Review the word, not the reading. If you see 食べる and recall たべる, you have learned the reading with its job.

What the Kanji Guide pages show

Every kanji page lists its on and kun readings and example words with their readings and meanings. For example, the page for 山 shows サン and やま, with 火山 (かざん) and 山 (やま) as examples. It is not a quiz on readings. It lets you attach sounds to words as you go.

For the shape and meaning, the picture story does the work. Readings and stories complement each other, and the combination is why a new kanji takes only a few minutes.

A short exercise

Pick five kanji you have learned, for example 山, 日, 人, 休 and 食. For each, write down one word with the on reading and one with the kun reading, then say them aloud. Compare with the kanji page. That ten-minute exercise teaches you more than a reading chart.

Where to go next

If you are new to kanji, start with how to learn kanji from zero. To see readings across a full level, open the JLPT N5 kanji list. And you can try the app, where readings and example words sit under every story.