Why do you want to go for CO certification and not for FI certification?
You should start with making a plan for preparation. Think about how much time you have for preparation and how many hours per day you can study. Then get the syllabus for the certification (here is the syllabus for SAP CO: https://training.sap.com/v2/certification/c_tfin22_65-sap-certified-application-associate---manageme...) and schedule which topic you are going to study when. The syllabus also indicated the importance of each topic with '+' signs. Make sure to allocate more time for topics with '+++' importance.
When you are done with the theory, test your knowledge with sample questions. On the link I gave you can find some plus you can also search internet for them. This practice will show you how well you are ready for the test and which areas need more studying effort.
Good luck with your certification!