Most kanji have more than one reading, and the word decides which one. Learn the patterns, then try the quiz, from easy to very hard.
Pick a kanji · most readings first · official Jōyō list
Kanji came to Japan from China over several centuries. Each wave brought the Chinese pronunciation of its time and place, and Japanese kept the older sounds alongside the newer ones. Those borrowed sounds are the on'yomi. For example, 生 is セイ in 先生 but ショウ in 一生. Japanese also attached its own native words to the characters, and those are the kun'yomi.
Sounds also shift when words join: ひ becomes び in 日曜日, ひと becomes びと in 人々, and ふん becomes ぷん in 三分. That is why the readings listed in a dictionary don't always match what you hear.