This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of old? It's also old, ancient, or out-of-date.
This word is used for things like clothes, food and friendships, but not for people! So a 古い友人 (old friend) is someone you've been friends with for a long time, not an elderly friend.
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.