When I meet someone for the first time, I tightly grasp their hand.
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means grip and the verb vocab version is to grasp.
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.