Scientific American created this helpful explainer of how the chain of Hawai'ian Islands formed. The tectonic plate is moving northwest over a magma hot spot in the earth's mantle. In fact, there's a new Hawai'ian island named Loihi forming underwater right now.
In other words, the northwesternmost island in the chain is the oldest, and they get progressively younger to the southeast. Loihi will be southeast of the current youngest, the big Island of Hawai'i. Don't book your travel to Loihi yet. It won't emerge for another 200,000 years or so, and it will be a bit toasty at first. It will also have hydrochloric gas and what they call Pele's hair, or tiny shards of volcanic glass, swirling about where the lava hits the water.