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:
After. Everything happens after. It's a cause and effect chain. One thing happens, and then another thing happens after. This is life. This is the universe. Everything is after. It's an automatic (あと) cycle that just keeps going and going and going.
Why don’t we have some tea together later?
Don't make me say, "I told you so," later.
After the movie, we headed across town to watch it again in another theater.