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:
There's something inside and you wanna get to what's within. Look inside. Ooh, cheese (うち)! Your favorite. Ooh cheese, how I love you.
Please paint the inside of the box pink.
There is an enemy within.
The thoughts I hide inside myself were spelled out in my diary.