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.
My little brother doesn’t watch TV.
That young man with blue hair is my younger brother.
I ended up going to the hospital last night because my younger brother threw a pool ball at my head.