Jisho

you need to focus on your vocabulary studies for that

one knows how a kanji is read in a specific word by knowing the word in question

Firsr of all, learning readings through vocab is absolutely one of the best approaches imo, as the person above me already said.

Generally it's noticable (to me atleast) that standalone (single)kanji words like 筋 すじ are more often spelled with their kun than their on yomi.
Kanji in words that contain okurigana like most verbs, e.g. 帰る are almost always spelled with their kun yomi(s).
Kanji in compound words without okurigana are more often spelled with on yomi × on yomi than in other ways, e.g. 必要 ひつよう. Of course there are exceptions. Sometimes both are possible as well, like with 白鳥 はくちょう/しろとり. But usually there's a preferred spelling in those cases.

By extension, if you know how some kanji are usually read, you can also read words whose meanings you don't know. That happens to me quite often, as most of the commonly used kanji don't posess a billion readings like 生 does for example.

Just like in english, how do we know when is "-eat" pronounced like "meat/wheat" or "sweat/great"? We don't.
You can guess, but always check the first time you see a new word.
Admittedly, if you know a word's history/etymology, you can guess more accurately, like how almost all the words related to Buddhism is pronounced with the 呉音 (go-on) type of On-yomi instead of 漢音 (kan-on) (those are the 2 main ones, that's why most kanji got 1 or 2 on-yomi and rarely 3 or more).
But then again, those knowledge are for the fun time, after mastering the basics, not for learning them. Keep learning vocabs and dont try too hard guessing them.

Also never forget that the language came before the script (that goes for both native Japanese words and the many loanwords). Then maybe it's clear that there's not necessarily much to gain by trying to rote memorize supposed readings of single characters.