Sign up for Divemaster Online - PADI's eLearning option - to start now. You can work through eight knowledge development sections using a web-based system that lets you learn at your own pace. You also have access to an online version of the Divemaster Manual for reference during and after the course.
Another option is to study by reading the Divemaster Manual and watching the Divemaster Video (a book and DVD package). Visit your local PADI Dive Center or Resort to enroll in the course and get your Divemaster Crew-Pak, which also includes other reference materials - like the PADI Instructor Manual and The Encyclopedia of Recreational Diving - that you'll need during the course.
Consider taking Dive Theory Online, another PADI eLearning® program, that takes you step-by-step through dive physics, physiology, skills, equipment and environment, plus a Recreational Dive Planner (RDP) review. By successfully completing Dive Theory Online, you can get credit for half of the Divemaster Final Exam. Your PADI Instructor can explain how this works when you meet to schedule knowledge review sessions along with your waterskills exercises, workshops and practical assessments.