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:
Someone yells out "NEXT!" because the first thing didn't work on you. That was a single guillotine. Now they are going to try two guillotines (つぎ) and see how that works on your thick neck. NEXXXXT!
Let’s catch the next train.
I'll let you know once we decide on the next plan.
The next destination on this bus tour is Michael Jackson’s house.