In order to take the exam to become a Certified CMMI® Associate you must have either attended an official "Introduction to CMMI" course or completed the "Fundamentals of CMMI Elearning" course.
The introductory CMMI courses are model-specific (each course covers one CMMI model) and are offered in-person, directly from the CMMI Institute and by many CMMI Partners, at locations and sites around the world. (Prices may vary.) To find a CMMI Partner and inquire about training, search the Partner Directory.
The "Fundamentals of CMMI Elearning" is an online course covering all three CMMI models and is offered for $500. (Note: You can bundle the Fundamentals of CMMI Elearning course together with the CMMI Associate Certification Exam for $675, at a savings of $75.)
For more information on the CMMI Associate Certification exam, see here.