The easiest way to get to the Hot Waterfall is as part of a pre-arranged day trip. These usually include a visit to the Emerald Pond as well, which isn't far away. Most hotels in the area will have a tour desk or travel service offering one or more trips like this.
In theory you can make your own way there by a rented car or motorcycle, but be forewarned that the route is not well-signed in English.
Admission to the Hot Waterfall is 90 Baht (2.79 USD). If you book a day trip, the admission fee will usually be included in the price.