Its important to remember that kanji and vocab are two different things. Kanji make up vocab, and sometimes its only that single kanji, but the concepts are different. Take 人, for example. The kanji means person and can be read as じん にん or ひと. So let me ask you, how do you say “person” in japanese? The answer じん or にん is 100% wrong, as the word for person can only be read one way: ひと.
rfindley has a pretty good example of this that imma steal.
What hes saying is that kanji, which make up words, can be read in multiple ways (much like letters of the english alphabet). The words they make up, however, have exactly 1 (in most cases) reading. Its important to familiarize yourself with how vocab words are read.
In the event that you think that you can skate by and people will understand you anyways, allow me to provide you with an experience I had. 2 years ago I was in Arashiyama and was asking for directions and I pronounced 北 as ほく. Got the blankest stare and I could not figure out why for the life of me. About 30 minutes later I had realized I switched the readings. ほく does not mean north. It is the onyomi reading for a kanji we have assigned the meaning “north” to, but it is not the word for north.