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: You go to grasp someone. You really need them. You are soooo needy (にぎ). Except, you can't really pronounce "d" sounds very well, so you say にぎ.
Since you make rice balls by pressing rice, it's called "onigiri."