This is one of the best hikes on Maui. If you're doing a one day trip on the Hana highway and enjoy hiking, this is your final destination.
The Pipiwai Trail is at the Kipahulu section of Haleakala National Park, where the Oheo gulch is. Signs at the park will point out the trail head, and the rangers will be able to give you detailed information about the hike, current conditions etc.
The trail is less than 4 miles in length and takes approximately 2.5 hours if you don't stop too often. If time is an issue, consider hiking the first 2/3 of a mile to Makahiku falls. The trail ends at the impressive 400 ft high Waimoku falls.