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:

Touch something that's warm... too warm! You pull your hand back and go "AH ta ta! (あたた). People make this noise when they touch something that's hot, I hear. It's almost like saying "hot hot hot" while dropping the "h" sound.