You know this is a verb because it ends with an う sound. The kanji for this word is right or success, so what's the verb form of that? Turns out it's a ton of things. But it's mostly for when things go right. As in to be right or to be on target.

当たる is intransitive, so it doesn't take a direct object. Your answer is right, or your shot is on target. See? No direct object.

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 to remember the あ portion.

When your guess turns out to be right, you can't help but shout, "Ah (あ)!" That "Ah" is your victory cry.