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 look at your younger brother. He's a scheming one. He's smiling up at you. Why? Because he's lit a firecracker on your foot. It explodes, sending your toe flying. You yell "OH. Toe? TOE!" (おとうと) as you realize what's going on.