If you want a charter closest to where you are staying, look to Lahaina Divers or Extended Horizons, both in the Lahaina Town area. If you don't mind getting up very early and driving down to the Kihei area, check out B&B, Mike Severns, and Ed Robinsons for charters. There are far more ops that go out of the Kihei area, but these seem to be the 3 that rank the highest.
Best recommendation is take a charter out of Kihei to dive Molokini crater and book a charter out of Lahaina for Lanai dives. If you've never done any diving in Hawaii, keep in mind that the trade winds pick up around noon and conditions deteriorate as the day goes on. Morning diving is preferred to afternoon, especially the charters out to Molokini and Lanai.