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:
You walk up to the tree. You have to get inside, because it takes you someplace secret. So, you take out your key (き) and put it in the slot, opening the door in the tree so you can slip inside.
There is a cherry tree in my garden.
My American friend's house is made of wood and bones.
It wasn’t my first time falling from the apple tree.