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: You look at your garden. It doesn't meet your expectations so you decide it's time to get a newer (にわ) garden. Tear it all down, folks! Getting a newer garden, here!
I'm gonna level the ground in the yard today.
I grow vegetables in our garden.
I have a beautiful garden but my neighbor’s chickens keep eating my flowers.