Since this word is made up of a single kanji, it should use the kun'yomi reading. When learning the kanji, you didn't learn that reading, so here's a mnemonic to help you with this word:
Imagine a giant heart sitting in a rowing boat. It's also drinking cocoa. Focus on all three of these parts and associate them into one story / memory. Once you've done that, sing the "cocoa row" (こころ) song! I don't know what that song is, but I want you to make one up and actually sing it, out loud. If you do, I bet you won't forget this word's reading too easily.
My heart constricted with longing.
I couldn't help shouting in my mind.
I wouldn’t know what to do if Anpanman had a cruel heart.