http://www.onespaworld.com/
Steiner recruits all around the world. They hold specific dates for interviews at different locations around the globe. You'll send in an application online. Their website is great and will answer many of your questions.
Their most recent upcoming interview in the states is in March 2012 in Miami. There may be changes, or possibly more interview dates added.
You must have the proper education for your specialty. They employ hairdressers, beauty therapists, massage therapists, fitness trainers, nail technicians, barbers, acupuncturists, and medispa doctors. I'm not sure if they are still hiring for receptionists, but their site says they are.
Look over the information they provide, this will answer lots of questions and begin the process of working on board! It also makes working on the ship feel so much more like a reality.