Since this word is made up of a single kanji with some hiragana attached, it's going to use the kun'yomi reading. You learned the on'yomi when learning the kanji portion, so let's use a mnemonic to help you with the reading of this word:
If you're old, you're also full (ふる) of memories. And you're probably full of old secrets. And you're full from old people food.
Those clothes are old, so I don’t need them anymore.
If you hold onto old stuff, you can't get anything new.
I found an old pair of underwear in our garage.