Since this word is made up of a single kanji, it should use the kun'yomi reading, and that is its most common reading. When learning the kanji, you didn't learn that reading, so here's a mnemonic to help you:
In your mind, the only person who can wear a crown is Bill Murray. He asks you, "Can I wear the crown?" and you say "Yes you can, Murray (かんむり)". Murray goes ahead and puts on the crown. He's movie royalty, after all.
Only the king can wear this crown.