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 just got beaten up because you're weak. Then the bully asks you: "Yo! Why (よわ) you so weak?"
I am not a morning person.
Yui is physically weak, but she is a person who understands others' pain.
Even superman gets really weak when he gets the flu.