Since this word is made up of a single kanji, it should use the kun'yomi reading. When learning the kanji, you didn't learn that reading, so here's a mnemonic to help you with this word:
If you remember in the radical/kanji mnemonic, you kick someone with your foot. But what happens after you kick them? Turns out they were made of fire, and you just burned your foot off, turning it to ashes (あし). Imagine this happening.
I have a tattoo on my leg.
When I was a kid, I thought hands were feet and feet were hands.
If I cut off my foot it will grow back soon, right?