Since this word is made up of a single kanji, it uses the kun'yomi reading. Here's a mnemonic to help you to remember it:

There is a man. Think of the manliest man you can. He works in an auto shop. As a child, he was an auto 子 (おとこ), working at the auto shop from an early age. This turned him into a man very quickly.

Like other mnemonics that aren't lined up quite perfectly, your goal here is to get close enough to remember the true reading. Step, step, step, jump!