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: This has the same reading as 暑い, so if you've learned that you'll know how to read this word as well. If you haven't run into that word, you've probably run into the kanji 暑 at least. That uses the reading あつ. Two similar concepts, but there are two kanji to differentiate, which gets confusing.
Please heat the bath to forty-one degrees.
I'm planning to brew some tea. Would you prefer hot or cold?
"Please eat it while it’s hot." "Thank you, but... well, it’s so hot that we can’t eat it yet."