The best thing to do to start is go to the page for the cert exam: https://certification.adobe.com/certification/customer-journey-analytics-business-practitioner-profe...
It has links to the exam info, practice tests, a prep guide, and other resources.
If you want other sources, Eric Matisoff just published a course on LinkedIn about CJA https://www.linkedin.com/learning/adobe-customer-journey-analytics
Studying all the materials isn't enough though, make sure you get lots of hands-on practical experience using CJA. Using the tool every day is better than studying any guide (doing both is best). Also talk to other users, ask questions about features you don't understand, and read others questions and comments.
