First to make sure you know your way around the platform and practice a few use cases. For example, build a report, create basic and more complex components, like derived fields or metrics from a dimension and vice versa.
Read documentation when you encounter issues. For example, when creating a derived field, you might get “No Value” or perhaps it doesn’t work as you expected. Read and try again.
Go over the training for certification for CJA.
Take any practice tests available and search any wrong answers to understand what you did wrong.
Repeat.
Mistakes provide the best opportunities for learning. Although this is no guarantee that you will pass. You should only take the test if you’ve really had the chance to dig into the platform and practice working on your organization's use cases.