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!
Who is that man?
I plan to go play pachinko with my guy friends today.
I developed quite a rapport with a man I didn't know at a fireworks show.