Don't learn Kanji and their reading, learn the whole words (and if you have to learn a reading for each Kanji, learn the onyomi). Onyomi are a bit like foreign (latin, greek) words in English: You put them together to construct a new word. So you'd construct "biology" from the greek words for life (bios) and science (logos). If English would use a system similar to Kanji, then you'd write 生学 and read it "biology". (Biology is actually 生物学 in Japanese, but I'm just trying to explain the principle). But if you'd see the Kanji for "life" followed by an ending, e.g. "he 生s", you'd read it "he lives", just as in Japanese 生きる "ikiru". And even if there's no ending, you'd read "you 生" as "you live"; that can happen, too. Japanese imported a lot of Chinese words, just like English imported Latin and Greek words, and these words use Onyomi, the Japanese variant of the Chinese pronounciation. The original Japanese words just got assigned Chinese characters by meaning, because the Japanese had no writing system of their own, and these are the Konyomi readings. In many cases the same native Japanese word can even be written with different Kanji, to express different aspects of the basic meaning. So learn the words themselves, and you'll be able to figure out the reading.

On'yomi is the Chinese reading of a kanji, and Kun'yomi is the Japanese reading. There are lots of exceptions to which to use, but a basic way of reading kanji as a beginner/intermediate is to associate the kanji's perimeters. If it's attached to another kanji, then it's (most likely) On'yomi. If there is hiragana attached to the kanji, then it's (most likely) Kun'yomi.