Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
You aren't able to turn on the light with just one finger though, you need to use two (つ). Either you have really weak fingers, or that's a really strong lightswitch. Either way, you need two fingers to switch on the light.
Turn on the light.
I don't know how to turn on the radio.
Can you turn on the TV?